This review will discuss hormonal and psychological factors involved in the initiation and maintenance of maternal behavior during the postpartum period in rat and human mothers. Research on primiparous rats suggests that among the ways hormones act to promote maternal responsiveness are by increasing the mother's attraction to odors of young pups, decreasing her natural neophobia and fearfulness, and increasing the ease with which experiences obtained during initial mother-young contacts are retained. Long-term maintenance of elevated maternal responsiveness in the rat is not directly under hormonal control but instead depends on a minimal period of direct interaction with young during the early postpartum. In human first-time mothers, the onset of maternal responsiveness is also directly affected by their mood state as well as their attraction to infants; these factors are, in turn, influenced by the amount of prior experience women have had caring for young. While the role of hormones in mediating maternal behavior in women is still unclear, initial results indicate they may facilitate responsiveness during the puerperium.