The diurnal pattern of protein synthesis in skeletal muscle was studied in relation to the serum concentrations of glucose, insulin and cortisol. Twelve healthy volunteers were given an ordinary hospital diet at 4-hourly intervals. The concentration of insulin showed peaks in response to food intake, and that of cortisol decreased over the day. Muscle biopsies were taken 30 min before the first food intake and 1.5 or 3.5 h after a meal. Irrespective of the time intervals after food intake, the total ribosome concentration, percentage amount of polyribosomes and polyribosome concentrations remained unaltered, indicating that there were no alterations in the capacity for protein synthesis accompanying the feeding schedule. The results suggest that a muscle sample taken in the morning after an overnight fast is representative of the protein synthesis during the day as assessed by the ribosome concentration and the size distribution of ribosomes.