The effects of prolonged muscular exercise (swim in tepid water for 60 min) on blood glucose, plasma FFA and R-GH were studied in a group of normal rats and the effect on blood glucose and plasma FFA in a group of hypophysectomized rats. The data obtained showed that, whereas in normal rats plasma FFA rose after muscular exercise, plasma R-GH fell sharply. In Hypophysectomized rats, however, muscular exercise did not increase plasma FFA levels.