Cholinephosphotransferase [EC 2.7.8.2] activity of rat liver microsomes, with 1,2-di-0-[3H]acyl glycerol or 1-0-hexadecanoyl [U-14C]ethanediol as substrate, was inhibited by N,N-dimethylaminoethyl p-chlorophenoxyacetate (centrophenoxine). Inhibition progressed in a linear fashion with increasing drug levels and was complete at 30 mM concentration. It appears that the microsomal enzyme was largely affected by the drug itself because the hydrolysis products of centrophenoxine, viz., N,N-dimethylaminoethanol and p-chlorophenoxyacetic acid, were less inhibitory.