Data are compiled on all reported Saimiri sciureus births; 2 deliveries -1 normal and 1 breech presentation-are described in detail. Compparisons between the squirrel monkey and other primate species are made on as many aspects of pregnancy, delivery and early post-partum behavior as possible. The gestation period in the squirrel monkey is 24-26 wks. During this period the pregnant female'' s weight increases by 27-57%. She may remain the object of sexual advances by the male, but in the last 12 wks of pregnancy she displays no sign of sexual receptivity. Feeding, gross activity and other behavioral patterns are variable. There is no reliable sign of impending labor, and labor is very short. Birth of the infant normally occurs from a cephalic presentation. The mortality rate for breech presentations of monkeys in captivity is about 90%. In the multiparous squirrel monkey, the duration of labor (about 1 h) and the number of contractions (10-12) are small, even as compared with other non-human primates. In a primiparous female, labor lasted about 1 1/2 h and involved 350 contractions. A more complicated delivery lasted about 60 h and involved more than 1000 contractions. Labor usually begins between dusk and dawn. If the delivery has not occurred by daybreak, the contractions cease and recommence at sundown. The infant-maternal weight ratio at birth is 12-19%. The infant is able to climb from birth, so that, while the mother possesses a number of behavior patterns capable of assuring support for many days even to a totally unresponsive infant, her supportive role in the postpartum period under ordinary circumstances is minimal. Behavior toward the placenta is quite stereotyped. As in other non-human primate species, manual extraction of the placenta occurs, but it is an inconstant finding. Severance of the umbilical cord is very uncommon. Usually the mother eats the placenta down to the cord; the cord itself she may chew or leave to dry and slough. The influence of social factors on the successful completion of different stages of reproduction is discussed. In this species the large majority of conceptions in captivity have occurred in groups of animals which have been together for 9 months or more. Most squirrel monkey infants which have been born in captivity and failed to live to maturity have died early in the postpartum period. Maternal aggression, maternal rejection, and the aggressive curiosity of other animals have contributed most to the high mortality rate. In some cases, females separated from their living groups have eaten their newborn infants. In others they have abandoned the infant or refused to let it nurse. The vocal behavior of the pregnant female and her living group was studied. Frequency spectrograms are presented for 4 vocalizations characteristic of the female in labor. The patterns of group vocalizations are analysed for successive stages of labor, delivery, and the postpartum period.