A comparative study of the placenta and the newborn infant at Rio Pallanga (altitude 15,100 feet) and Lima (500 feet) is presented. Three parameters were analyzed, the weight of the newborn infant, the weight of the placenta, and the ratio of the weight of the placenta to the weight of the newborn infant (placental coefficient). The mean values of these parameters were analyzed according to Fisher's t test. The results showed that: (1) at high altitudes, the newborn infants weighed significantly less than those born at sea level; (2) the placentas at high altitude weighed more than the placentas at sea level; (3) the value for the placental coefficient at high altitudes was significantly higher than at sea level; (4) the number of cotyledons in high-altitude placentas was half that of sea level placentas, indicating a lesser degree of septation and probably a greater proportion of functioning parenchyma; and (5) no differences were found in the volumes of the placentas. The physiologic role of the placenta at high altitudes is discussed. © 1970.