In this study, protein efficiency ratio and net protein utilization together with the kinetic estimates of protein turnover were used to compare the effect of different protein and fat sources in healthy rats. Male Sprague-Dawley CD rats were pair-fed different diets for 14 d. All diets were isonitrogenous and isocaloric, containing 10.4% protein, 10.9-11.4% fat, 31.9-32.8% carbohydrate and 43.5-44.5% moisture (wt/wt). After 14 d of feeding, protein efficiency ratio, net protein utilization, weight gain, intake, fat and protein content in the whole-body and fractional synthetic rates in various tissues were determined. Animals given diets containing medium-chain triglycerides (MCT) demonstrated decreased weight gain and fat content compared to the pair-fed controls receiving long-chain triglycerides (LCT). No difference was seen in protein content, net protein utilization and fractional synthetic rates in the liver and whole body of these MCT-fed rats when compared to those given LCT. Protein efficiency ratios in both of the MCT groups fed MCT + casein and MCT + soy protein were lower than those in the groups given LCT + casein. Although this study did not include a group for LCT and soy protein, these results suggest that MCT reduces the fat deposition without affecting the whole-body protein content. This may have implications for the treatment of obesity. Secondly, the protein efficiency ratio may not be a useful indicator of dietary protein quality when the fat source is MCT.