The metabolism of high density lipoprotein cholesteryl esters (HDL CE) was studied in the pony, an animal species without plasma cholesteryl ester transfer protein (CETP) activity. Studies were done in ponies fed a low- (1.5% fat, w/w) and a high-fat diet (11.5%, w/w fat). The ponies fed the high-fat diet had higher plasma HDL CE concentrations (1.08+/-0.15 vs. 0.84+/-0.11 mmol/l, mean+/-S.D., n=6, P<0.01) and plasma lipoprotein lipase (LPL) activities (14.3+/-4.0 vs. 5.7+/-3.4 micromol free fatty acids (FFA)/ml per h, P<0.05) than those on the low-fat diet. Plasma triacylglycerol (TAG) concentrations were lower on the high-fat diets (0.129+/-0.043 vs. 0.180+/-0.050 mmol/l), but these differences were not statistically significant. There was a negative correlation between the levels of plasma TAG (r=0.598, P<0.05) and VLDL CE (r=0.658, P<0.05) on the one hand and the HDL CE concentrations on the other hand. The transport rates of HDL CE were not significantly different between ponies fed high-fat (0.029+/-0.008 mmol HDL CE/h per l plasma) and those fed low-fat diets (0.024+/-0.004). HDL CE were transferred to low density lipoproteins (LDL) and we calculated that the percentage of LDL CE derived from HDL was 0.69+/-0.13 in the ponies fed the low-fat diet and 0.53+/-0.05 in the ponies fed the high-fat diet (P<0.05). The results of these in vivo studies suggest that in ponies, similarly as reported in rats and pigs, HDL CE can be transferred to LDL despite the absence of plasma CETP activity, and that the magnitude of this transfer is related to the levels of HDL CE as induced by the amount of fat in the diet.