The effect of fumonisin B1 (FB1) on protein and lipid synthesis was evaluated in primary rat hepatocytes. FB1 did not affect incorporation of [3H]leucine into hepatocytes at either non-toxic (150 microM) or cytotoxic (500 microM) concentrations indicating that protein synthesis was not affected. However, FB1 significantly (P < 0.01 to P < 0.0001) inhibited incorporation of [14C]palmitic acid into hepatocyte cultures implying that lipid synthesis was altered. Incorporation of the radiolabel was significantly (P < 0.05 to P < 0.0001) lowered in triacylglycerol (TAG) and sphingomyelin fractions and increased in phosphatidylcholine (PC) and phosphatidylethanolamine (PEA) in both FB1 concentrations. The incorporation pattern of [14C]palmitic acid closely resembles the changes in phospholipid levels in the treated cells. The sphingolipid, sphinganine (Sa), was significantly (P < 0.0001) increased in treated cells but there was no significant difference between the toxic and non-toxic dose levels implying that the increased Sa level alone is not responsible for the in vitro toxicity. FB1 significantly (P < 0.01 to P < 0.001) decreased the level of free cholesterol within the cell, resulting in an increased PC:cholesterol ratio suggesting a more rigid membrane structure. Subsequent studies on the fatty acid (FA) profiles in PC and the neutral lipid, TAG, indicated that FB1 significantly (P < 0.05 to P < 0.0001) increased the levels of the polyunsaturated FAs C18:2n-6 and C20:4n-6 at both concentrations. The FB1-induced changes to cellular membranes, specifically those related to FA changes in the major membrane phospholipids, and the altered FA content of the hepatocytes are likely to be key events in explaining the cytotoxic effects and altered growth responses induced by fumonisins in primary hepatocytes.