Propionate is a short-chain fatty acid formed in the colon and supposedly involved in the cholesterol-lowering effect of soluble fibre. To explore the underlying mechanism(s) of this fibre action, we have used human hepatocytes in primary culture to study the effects of propionate on hepatic lipid synthesis. Initial experiments with mevalonate and mevinolin, a competitive inhibitor of hydroxymethylglutaryl (HMG)-CoA reductase (EC 22.214.171.124) were performed to evaluate basic regulatory mechanisms in these cells; results were compared with those obtained with rat hepatocytes. Incubation for 24 h with mevalonate caused a similar, concentration-dependent inhibition of [14C]acetate incorporation into cholesterol in human and rat hepatocytes. Likewise, mevinolin (100 mumol/l) inhibited the formation of cholesterol from radiolabelled acetate by about 80% in cells from both species. Propionate inhibited cholesterol as well as triacylglycerol synthesis from [14C]acetate with a similar concentration-dependency in rat hepatocytes. Fifty percent inhibition was obtained at a propionate concentration of only 0.1 mmol/l. This propionate-induced inhibition was not affected by a 100-fold excess of unlabelled acetate. Human hepatocytes were much less susceptible in this respect: propionate concentrations of 10-20 mmol/l were required to obtain similar inhibitory effects in these cells, i.e. values greatly exceeding reported portal propionate concentrations in humans. The results suggest the existence of differences in the regulation of hepatic cholesterol (and triacylglycerol) synthesis between human and rat liver cells. These results do not support the hypothesis that the fibre-induced decrease in plasma cholesterol concentration in man is mediated by a direct effect of propionate on hepatic cholesterol synthesis.