Parameters of skeletal muscle protein synthesis was studied in vivo and in vitro. The experimental design was previously described. Muscle humidity, protein and DNA content are not modified by vitamin A deficiency. RNA and free amino acids are modified. Glutamine, serine and threonine level decrease while phenylalanine and leucine increase in vitamin A deficients. After a single intra-peritoneal injection of (14C) leucine, the incorporation of radioactivity into total ribosomes and postribosomal supernatant protein was studied. The rats were killed 20, 40, 60 minutes and 2,6,10 days after injections. The half time for radioactivity accumulation was four times higher into total and ribosomal protein, two times higher into postribosomal supernatant protein in vitamin A deficient rats than in well-fed rats. The decay of acid soluble radioactivity was similar in the two groups, although protein synthesis was lowered by vitamin A deficiency. It was then supposed that leucine, enters into other metabolic paths. Sedimentation analysis in sucrose gradients revealed a lower proportion of higher ribosomes species in muscle of deficient rat than in normal ones, but ribosomes from deficients were more active for protein synthesis in vitro than ribosomes from well-fed rats. The vitamin A deficiency decrease the muscle protein synthesis and lead to important changes on in vitro and in vivo ribosomal activity.