Thirty male rats were randomly assigned to one of three dietary groups in which the source of dietary fat was either a mixed oil, maize oil or fish oil. Effects of dietary fatty acid composition on in vitro rates of [U-14C]glucose incorporation into hepatic total lipids and into hepatic triacylglycerol were measured under basal, insulin (4 nM)-, gastric inhibitory polypeptide (GIP; 6 nM)- and insulin + GIP (4 nM + 6nM)-stimulated conditions. Effects of the three diets on postprandial plasma triacylglycerol, cholesterol, insulin and GIP concentrations were also measured. The fish-oil diet decreased rates of basal glucose incorporation into hepatic total lipids (P < 0·05) and hepatic triacylglycerol (P < 0·01) compared with the mixed-oil diet. The presence of insulin and GIP in the incubation medium stimulated glucose incorporation into hepatic total lipids in the maize-oil (P < 0·01) and fish-oil groups (P < 0·05), as well as into hepatic triacylglycerol in the maize-oil group (P < 0·005). In addition, the fish-oil diet decreased postprandial plasma triacylglycerol levels compared with both other dietary groups (P < 0·05 both cases), and the mixed-oil diet markedly increased postprandial plasma insulin levels compared with the other dietary groups (P < 0·001).