A NUMBER of otherwise pharmacologically inactive compounds are known to prolong sleep induced by barbiturates in mammals1. The most widely investigated of these is `SKF 525A' (2-diethylaminoethyl 2,2-diphenylpentanoate), which prolongs the action of barbiturates and other drugs by inhibiting liver microsomal enzymes which degrade a wide variety of drugs1. When applied to insects, `SKF 525A' synergizes pyrethroid insecticides and antagonizes the toxicity of the organophosphorus poison, malathion2. In these respects, `SKF 525A' mimics the action of commonly used insecticide synergists such as piperonyl butoxide2. Although piperonyl butoxide and other chemically related synergists have been included in insecticide formulations for many years, their effects on drug activity in mammals have not been examined. In this communication the effects of two insecticide synergists on the hypnotic action of barbiturates are described.