Using the weak hepatocarcinogen thioacetamide, we found a 73% decrease in microsomal CTP: Phosphocholine cytidylyltransferase (CT) activity after two months of treatment. We investigated the effect of different effectors on this enzyme activity. Results show that incubation of liver homogenates of treated animals in the presence of either oleate or spermine restored control values of microsomal activity. Incubation of thioacetamide homogenates in the presence of cAMP showed no changes in CT activity, while incubation of control and thioacetamide homogenates in the presence of Ca2+ induced an activation in microsomal activity, although of less intensity in thioacetamide homogenates than in control preparations. Results suggest that thioacetamide alters the regulation of cytidylyltransferase activity.