3-Methylhistidine excretion in vivo and in vitro was monitored in hypervitaminotic and pair-fed control rats. Feeding with excess of retinyl palmitate (40 000 i.u./day per 100 g body wt.) significantly increased urinary 3-methylhistidine and creatinine output during a 4-day treatment interval. 3-Methylhistidine release from perfused rat hindquarters was also elevated after 5 days of vitamin treatment. To determine whether the adrenals were involved in mediating the above response, a study was conducted on adrenalectomized and sham-operated rats. Excessive vitamin A intake stimulated 3-methylhistidine excretion in vivo and in vitro in both adrenalectomized and sham-operated animals, thus suggesting that the vitamin A-induced acceleration in myofibrillar protein breakdown was not mediated by the adrenals. In both groups of rats, vitamin A treatment had no effect on the rate of protein synthesis, on the basis of incorporation in vitro of [3H]phenylalanine into muscle protein. Additional studies revealed that the addition of excess retinol to the perfusion medium (10 i.u./ml) had no significant effect on the rates of 3-methylhistidine release or [3H]phenylalanine incorporation in vitro. Finally, high doses of cortisol (7 mg/day per 100g body wt.) administered to intact rats for 5 days significantly increased rates of 3-methylhistidine excretion, both in vivo and in vitro.