Guinea pigs demonstrate “precocious” physical and functional development, with newborns displaying open eyes, hair, self-feeding, and temperature regulation. In addition, morphologic lung development is precocious in the guinea pig, with advanced alveolarization taking placein utero. To explore whether pulmonary surfactant development is also advanced, and at what stage prematurely delivered guinea pigs are capable of survival, we delivered fetal guinea pigs at 2- to 3-day intervals from day 49 of gestation to day 69 (birth). These were examined for chronologic changes in lung morphology, lung tissue disaturated phosphatidylcholine, phosphatidylglycerol and glycogen content, and serum glucocorticoid and thyroid hormone levels. Other prematurely delivered guinea pigs were given brief postnatal resuscitation and their survival noted. We confirmed advanced morphologic lung maturation, yet found that surfactant development, with antecedent hormone peaks and glycogen depletion, occurs during the final 10-15% of gestation. Lung biochemical development is thus “on time” in the guinea pig, rather than “precocious” compared to other frequently studied laboratory animals. In addition, >50% of fetal guinea pigs are capable of survival by 8 days prior to term, well in advance of premature survivability in other small-sized species. (Pediatr Res21: 427-431,1987)