The purpose of this work was to determine whether alterations in the lipid composition of rat liver microsomal membranes existed during thioacetamide-induced injury prior to the development of hepatic cancer and biochemical mechanisms involved. Rats were injected intraperitoneally with (50 mg/kg body wt per day) thioacetamide or diluent for 8 days. Liver homogenates and microsomal membranes from liver homogenates were obtained. Incorporation of [32P]orthophosphate into whole liver lipids and hepatic microsomal lipids was evaluated 75 min after isotope administration. These determinations were made after two separate periods of treatment (3 and 8 days). Activity of sphingomyelin synthase was assayed in rat liver homogenates as well as in the purified microsomal fractions. Results demonstrated a maintenance of liver and hepatic microsomal contents of phosphatidylcholine during thioacetamide-induced injury even when the biosynthesis of this glycerophospholipid in both liver and their microsomal fractions appeared decreased. Also observed was a considerable increase of microsomal sphingomyelin, as well as an increased hepatic biosynthesis of sphingomyelin caused by thioacetamide treatment. The microsomal sphingomyelin/phosphatidylcholine radioactivity ratio significantly increased. Sphingomyelin synthase activity in liver homogenate appeared stimulated. In conclusion, our data are consistent with a thioacetamide-induced increase in microsomal sphingomyelin by a stimulation of sphingomyelin synthase. Based on this and previous studies, accumulation of sphingomyelin in the microsomal purified fraction is associated with the number of thioacetamide doses and is an early event clearly detected prior to tumoral characteristics of hepatocytes.