Two vertebrate hypolipidemic agents, cholestyramine and niacin, affected the growth and development of Heliothis zea as well as the quantity of acylglyceride or sterol present in the larva. As the concentration of cholestyramine in the diet increased to 6.0%: the number of larval molts increased from 5 or 6 to as many as 7 or 8, the time required for the onset of pupation increased from 11 or 12 to 20 days, and the number of adults that emerged decreased from at least 70 to 0%. The growth and development of the insect may have slowed, at least in part, because this agent reduced the quantity of sterol and glyceride in the tissues of the larva. Niacin also affected the growth and development of the insect. As the concentration of niacin in the diet increased to 5.0%: the number of larval molts increased by 1, the time required for the onset of pupation increased to 21 days, pupal weight decreased significantly, but adult emergence was normal. The growth and development of the insect may have slowed, at least in part, because this agent caused sterol to accumulate in the hemolymph of the larva. A water-soluble component in the hemolymph also increased in the presence of niacin, but there was little or no change in the glyceride content. Further studies are warranted to determine the mode of action of these hypolipidemic agents in H. zea.