Abstract Fifteen boys and eleven girls between three and six years of age were examined at a day-care center and at home as regards their catecholamine and cortisol excretion. Behaviour pattern was assessed by the MYTH-scale, which measures the competitiveness, impatience-anger and aggression components of the type A behaviour pattern in children (Matthews and Angulo, 1980). The boys obtained higher type A scores and excreted more adrenaline and noradrenaline than the girls, while cortisol excretion did not differ between the sexes. In view of previous findings, the results suggest that sex differences in catecholamine excretion in children are induced by sex related differences in behaviour. This relationship in childhood could be of relevance for sex differences in catecholamine responses observed in adulthood. In both sexes, adrenaline excretion was significantly elevated at the day-care center compared with the at-home levels, indicating that mental arousal was greater at the center. In a separate part of the study, eleven new children were tested while they were adjusting to the day-care situation; it was found that only noradrenaline levels during the first week at the center were significantly elevated.