1. Single subcutaneous injections of nicotinic acid lowered the serum free fatty acid and triglyceride concentration in two non-fasting Rhesus monkeys.
2. Injections of nicotinic acid repeated daily for 2–4 weeks led to a substantial fall in serum cholesterol concentration, comparable with that obtained in humans given therapeutic doses of nicotinic acid by mouth.
3. The fall in serum cholesterol concentration was accompanied by a decrease in hepatic synthesis of cholesterol but there was no change in the faecal excretion of endogenous steroids.
4. The rate of synthesis of cholesterol, estimated from the faecal excretion of endogenous steroids and from the serum specific radioactivity curve, was such that a moderate degree of inhibition would have accounted for the observed decrease in the amount of circulating cholesterol during nicotinic acid treatment.