In this study we tested the hypotheses that higher fat diets will promote greater fat oxidation, and that medium-chain fatty acids (MCFA) will be more readily oxidized than long-chain fatty acids (LCFA). The oxidation of dietary MCFA (coconut oil labelled with C-14-octanoic acid) and LCFA (olive oil labelled singly with C-14-palmitic acid and C-14-oleic acid) was measured in a metabolic chamber. MCFA and LCFA were included in the respective diets at 150 and 300 g kg(-1). Polka dot grouper fed diets containing MCFA had a significantly higher (P < 0.05) respiration rate [2.4 mmol CO2 BW (kg)(-0.79) h(-1)] and proportion of radioactivity in the respired CO2 (49.5%) than those fed diets containing LCFA [1.2 mmol CO2 BW (kg)(-0.79) h(-1) and 12.8%, respectively]. The amount of dietary fat did not significantly affect either of these two response attributes. The radioactivity in faeces and regurgitated material was low with diets containing MCFA, but was significantly higher with diets containing LCFA, and increased with increasing LCFA content, suggesting a decrease in digestibility. These results show that MCFA provide this species with a more readily utilizable source of energy than LCFA. Moreover, raising the amount of lipid in the diet above 150 g kg(-1) did not increase the oxidation of fatty acids for energy.