The aim of this study was to assess basic information about adrenocortical activity and its correlates in newborns and infants until the age of 7 months. The circadian adrenocortical activity of 25 healthy infants (11 newborns and 14 3-7-month-olds) and their mothers was assessed from saliva. Additional information about infants' sleep and feeding was collected. In newborns, two-phase patterns of circadian adrenocortical activity were observed. Circadian patterns of adrenocortical activity seem to develop during the first 3 months. At three months adult-type patterns of circadian rhythm in adrenocortical activity were obvious. Stability of individual differences in newborns' cortisol levels was high. Correlates of infants' adrenocortical activity were found in sleeping and feeding patterns as well as in maternal adrenocortical activity. The development of the 24-h sleep-wake cycle paralleled the development of the circadian pattern in adrenocortical activity. The findings suggest that for an adequate interpretation of relationships between adrenocortical activity and behavior, information about basic cortisol levels and the temporal organization of feeding and sleeping must be considered.