Testosterone, estradiol, 170H-progesterone, and androstenedione (except in cord samples) concentrations were determined in cord sera (30 male and 14 female) and in peripheral sera from infants (121 male and 110 female), age 1 day to 2 years. Male and female cord serum levels of these steroids were not significantly different. In both sexes levels during the first week were lower than those in cord sera. In male infants serum testosterone and 170H-progesterone levels rose sharply in the second week of life, reached a peak at 1-2 months, and then declined to the range seen in later childhood by 6 months of age; male serum androstenedione and estradiol concentrations were higher during the first 2 months of life, but no distinct pattern of rise and fall was seen. In girls serum testosterone levels fell in the first week to the range seen throughout childhood; serum concentration of estradiol, androstenedione, and 17OH-progesterone in girls were markedly variable, with many values above the childhood range being seen, particularly in the first 6 months. These data provide further evidence of active Leydig cell function in male infants. They suggest that there is also ovarian secretion of sex steroids in some female infants in response to the elevated FSH and LH levels which are seen at this time.