1. Rates of protein synthesis in liver and muscle of 100 g male rats were measured in vivo at 1 h or 4 h after injection of 2.5 mg of corticosterone and compared with those from animals given carrier medium alone. 2. In post-absorptive rats, corticosterone for 1 h had no effect on either muscle or liver protein synthesis. After 4 h there was a decrease in both tissues, but this was only statistically significant in muscle. 3. In fed rats, rates of protein synthesis were higher than those in post-absorptive animals, but the effects of corticosterone injection were similar. 4. Re-feeding of post-absorptive rats led to an increase in muscle protein synthesis after 1 h and 4 h. At 1 h this increase was not inhibited when plasma corticosterone concentrations were maintained high by injection of the hormone immediately before feeding commenced, but at 4 h there was a small inhibition. 5. It is concluded that the action of corticosterone in depressing muscle protein synthesis is time-dependent and requires longer than 1 h to develop. The failure of the hormone to alter the response to re-feeding for 1 h in post-absorptive rats suggest that corticosteroids are not important mediators of the acute stimulation of muscle protein synthesis by food intake.