Infant rats received an i. p. injection of insulin, anti-insulin serum, streptozotocin, antiglucagon serum or dexamethazone. All substances except the antiinsulin serum, raised the plasma triglyceride level. Both antisera decreased plasma cholesterol levels, while streptozotocin, insulin and dexametazone caused an increase. The activity of 3-hydroxy-3-glutaryl CoA reductase in liver and brown adipose tissue changed inversely to the cholesterol level. However, small intestinal enzyme activity was increased by insulin administration inspite of the rise in plasma cholesterol.