There is a general uniformity in the hormones that regulate pregnancy among a large variety of mammals. There is also a division of maternal behavior into two main patterns: that shown by mammals that build nests for the altricial young and those that only briefly establish a birth site for the precocial young. Apart from this difference, the patterns of maternal behavior are quite similar, centering around nursing, and weaning follows a similar course in different species. Is there a corresponding uniformity in the factors which regulate maternal behavior? Too few species have been studied to establish any general rule about the hormonal basis for the onset of maternal behavior. In the main, however, the ovarian hormones probably play a major role in most species and prolactin appears to play a role in at least one species. Post-partum maternal behavior appears to depend mainly upon stimulation from the young and hormones may not be required to maintain it or be involved in its decline. The transition period between hormonal and nonhormonal regulation of maternal behavior may emerge as an important phenomenon in many species.