Growth and development of the infant pigtailed macaque (Macaca nemestrina) over the first 3 weeks of life have been characterized by the variables of body weight, blood pressure, heart rate, ventilation, blood gas tensions, and lung mechanics. The effects of prematurity on postnatal developmental trends were assessed at gestational ages of 135–145 days (0.80 of term) and 150–155 days (0.91 of term). There was no indication that cesarean section, restraint, or instrumentation had any significant influence on the measurements. Gestational age at delivery had no effect on the minute ventilation per kilogram body weight, the hematocrit, or the heart rate; however, body weight, tidal volume, respiratory frequency, arterial gas tensions, blood pressure, and lung compliance did vary with gestational age at delivery. Postnatal maturational changes in these variables were similar between term and premature animals. The data for infant macaques and newborn humans were compared. The newborn macaque appears to be an excellent model of human developmental trends (and/or disease state) over the first 3 weeks of life, though some potentially important differences have been found.