Effects of endogenous and exogenous polyunsaturated fatty acids (PUFA) on microsomal synthesis of docosahexaenoic acid (DHA) from eicosapentaenoic acid (EPA) in vitro were studied using rat livers. Liver microsomes prepared from three groups of rats which had been fed n-3/n-6 ratios of 0.01, 0.39, and 2.70, respectively, for 1 month, were incubated with [1-14C] EPA at 37°C for 30 min. There were no significant differences between the formations of docosapentaenoic acid (DPA) or DHA in the groups. Liver microsomes from rats fed a diet with an n-3/n-6 ratio of 2.70 for 3 weeks were then incubated with [ 1-14C] EPA in the presence of unlabeled DHA or arachidonic acid (AA). The additional DHA did not have any effect. However, a significant inhibition of DHA synthesis was observed by addition of AA (EPA/AA=5:2). These results suggest that the microsomal biosynthesis of DHA in rat liver is not susceptible to feedback inhibition.