Feeding a high-fat diet may have potential advantages by maintaining pH homeostasis during intense exercise; conversely, effects including elevated PCV and increased plasma concentration of total protein may indicate effects detrimental to performance by reducing perfusion of tissues and organs. Six horses were used to determine the effects of an replacement of starch (diet ‘0.00’) with 0.33, 0.67, 1.00 and 1.33 g soybean oil/kg bwt/day (diets ‘0.33’, ‘0.67’, ‘1.00’ and ‘1.33’) on venous and renal acid-base status and fractional clearance (FC) of electrolytes in a postprandial state but prior to exercise. Venous blood and urine were sampled for 3 days at the end of the 28 day periods, in each case 3 h postprandially. Throughout the study, the horses put on weight by 35 kg (P<0.05). From diet ‘0.00’ to ‘0.67’, blood pH decreased (P<0.05) and carbon dioxide tension increased (P<0.05). Simultaneously, the base and net acid-base concentration in the urine rose (P<0.05) but diuresis decreased (P<0.05) and, therefore, the daily excretion of bases and net acid-bases was only unsystematically affected. From diet ‘0.00’ to ‘1.33’, the FC of magnesium and chloride fell (P<0.05). These results suggested that the exchange of starch with up to moderate amounts of fat affects the venous and renal acid-base status of resting horses. The decrease in urine volume is most striking. Further research is needed to investigate the importance of these effects for sporting performance.