The purpose of these studies is to indicate the biochemical alterations in intrauterine growth produced by interference with placental circulation or the chemical ablation of the fetal pancreas. In a group of pregnant rhesus monkeys the injection of Streptozotocin (approximately 75 mg/Kg) directly into the fetus produced cytotoxic effects on the pancreatic B-cells. Five of nine surviving animals had significantly large adrenals and livers. Body weight was normal. Two animals were significantly small for gestational age and had normal adrenals and livers. The remaining endocrine organs were of normal size in all animals. The protein:DNA ratio in muscle was increased in four animals. The fetal insulin levels at term were normal as were the blood glucose values. Regions of pancreatic regeneration and B cell activity could be identified on histological section. In a second group of pregnant monkeys the interplacental fetal vessels were ligated at 100 days gestation producing a reduction in placental mass and a placental insufficiency. Nine of 16 surviving fetuses had significantly low birth weight (2 S.D. below the mean) for geatational age. The brain was least affected while the liver was most affected and the small animals had a high brain:liver ratio. Total DNA, RNA, and protein were significantly low in muscle and liver. The protein:DNA ratio and the number of nuclei (cell number) were significantly low in muscle. The percentage fat was reduced in the carcass as well as the total fat, protein and collagen.