With findings that are bound to rekindle the debate over its effects on children, two studies being published today build on evidence that those who spend long hours in child care may experience more stress and are at increased risk of becoming overly aggressive and developing other behavior problems.

One of the studies found that the more time children spent in child care, the more likely they were to be disobedient and have trouble getting along with others, though it suggested that factors like a mother's sensitivity to the child's needs could moderate that outcome.

This report is from the Study of Early Child Care and Youth Development, the largest long-term study of child care in the United States, which was undertaken by the National Institute of Child Health and Human Development, a branch of the National Institutes of Health. The findings elaborate on preliminary research that created a storm of debate when presented by the study's investigators at a child development meeting two years ago.

The other study found that in children younger than 3, levels of cortisol, a hormone associated with stress, rose in the afternoon during full days they spent in day care, but fell as the hours passed on days they spent at home. This study's researchers, from the Institute of Child Development of the University of Minnesota, had earlier found the same pattern in 3- and 4-year-olds.

Cortisol levels in the saliva of day care children were highest and rose most steeply in those judged by day care center personnel to be the shyest. "These children struggle in group situations and find them stressful," said the study's lead author, Dr. Megan R. Gunnar.

Dr. Gunnar said that while none of the cortisol levels measured were high enough to be considered signs of psychological trauma, they were nonetheless a cause for concern.

In a measure of how sensitive the topic of child care has become, the studies, appearing in the journal Child Development, are accompanied by nine commentaries from researchers around the world that put the findings into perspective and, in some cases, rebut them. The editors of the journal delayed publication of the studies for several months while they circulated the manuscripts to more than 1,000 child development experts and invited them to write commentaries.

"I think it was worth holding up publication of the research so that we could put it in context," said Dr. Lynn S. Liben, editor in chief of Child Development. "Child care is a controversial issue."

Unlike the University of Minnesota research, which dealt only with children in day care, the study by the National Institute of Child Health and Human Development followed more than 1,000 children in 10 cities who were in the full range of child care arrangements, including day care centers, preschools, care with nannies and care with relatives other than their mothers. The children's behavior was evaluated by their mothers, caregivers and kindergarten teachers.

The study, which began in 1991, found that the more hours the children spent in child care, the higher the incidence of problem behavior and the greater its severity.

Over all, about 17 percent of the children had above- average levels of problem behavior like disobedience and overassertiveness. Though their behavior fell within normal limits, children exhibiting such traits would be at risk of developing behavioral abnormalities, said the study's scientific coordinator, Dr. Sarah L. Friedman.

Most of the children in the study spent 10 or more hours a week in child care, and 10 percent spent more than 45 hours a week there. The correlation between quantity of child care and behavior problems remained even when other variables were taken into account, including the quality and type of the child care, the mother's sensitivity to her child's needs and the family's socioeconomic status. Indeed, the study found that the time spent in child care was linked more strongly with children's behavior than was the quality of care.

But while none of those variables entirely offset the negative effects that the study found, the mother's sensitivity and the family's socioeconomic status had a greater influence on children's behavior than did the amount of time spent in child care. Greater maternal sensitivity and higher level of family income and education correlated with better behavior in the children, the study found.

The commentaries published along with the two studies elaborated on some of the findings and challenged others. Susan C. Crockenberg, a professor of psychology at the University of Vermont, cited other research concluding that boys were more vulnerable than girls to negative effects of child care.

A few of the commentators argued that contrary to the findings of the National Institute study, the quality of child care mattered a great deal in fostering young children's social and emotional development. A commentary by several researchers led by John M. Love, a senior fellow at Mathematica Policy Research of Princeton, N.J., noted that the more time that infants and toddlers spent in Head Start programs, considered to be of high quality, the fewer their behavior problems and the greater their intellectual and language development.