The effect of insulin on adipose tissue triglyceride formation was investigated. Triglyceride formation was measured in the presence of [14C]-glycerol-3-phosphate, palmitate, ATP, CoA and Mg2+50 with adipose tissue homogenate as an enzyme source. Glycerolipid formation by the homogenates prepared from adipose fragments incubated in the presence of insulin for short time (90 min at 37 degrees C) did not appreciably differ from those incubated in the absence of insulin. However, tissue homogenates prepared from rats treated with insulin for 7 or 14 days showed significantly greater rates of glycerolipid formation compared to control animals. Streptozotocin treatment also resulted in increased rates of glycerolipid formation in adipose tissue. These results suggest that a factor common to insulin excess or deficiency may be responsible for the increased rates of adipose glycerolipid formation under these two experimental conditions.