Maternal responsiveness in primates has long been considered emancipated from endocrine factors and entirely dependent on experience and cognition. Here we report that group-living pigtail macaque females increased their rate of interaction with infants in the last weeks of pregnancy in correspondence with an increase in plasma levels of estradiol and progesterone. Estrogen treatment increased the rate at which ovariectomized rhesus females interacted with infants. This is the first evidence that steroid hormones influence maternal responsiveness in anthropoid primates. All untreated ovariectomized females and nonpregnant females interacted with infants, indicating that although estrogen can enhance responsiveness to infants, ovarian or pregnancy hormones are not necessary for the expression of infant-directed behavior in female macaques. The findings of this study suggest fundamental similarities, rather than differences, in the endocrine modulation of maternal responsiveness in primates and other mammals.