The effect of the lipidic composition of the diet on the proteic synthesis of the male Sprague-Dawley rats fed parenterally for four days has been studied. All animals received identical nitrogen and caloric intake, but the ratio of medium to long-chain triglycerides (MCT/LCT) varied: group 1, control (0/100); group 2 (30/70); group 3 (50/50) and group 4 (70/30). Hepatic and jejunal protein synthesis were determined with L-(1-14C)-leucine. In groups 1, 3 and 4 muscle protein synthesis was measured with L-(3,4-3H)-phenylalanine. In liver, there were no significant differences in the fractional synthesis rate among the groups. In jejunum, the control group showed a higher fractional synthesis rate with statistically significant differences among the groups. In muscle, group 3 presented the highest fractional synthesis rate with statistically significant differences (p less than 0.05).