The effects of a beta 2-adrenoceptor agonist, clenbuterol, on body weight and protein metabolism of gastrocnemius muscle, heart, and liver were studied in rats subjected to 50% food restriction or fasting. Food restriction by 50% for 7 days caused a complete cessation of growth and reductions in the mass, protein, and RNA content of muscle, heart, and liver. The ratio of RNA to protein content was also suppressed in muscle and heart, but not in liver. Fasting for 3 days caused loss of body weight (BW), reductions in the mass, protein, and RNA content, and the ratio of RNA to protein of gastrocnemius muscle, heart, and liver. Oral administration of clenbuterol (approximately 0.6 mg/kg BW/d) to food-restricted animals did not affect BW, but did increase in the mass, protein, and RNA content, and the ratio of RNA to protein of gastrocnemius muscle. The protein content of heart was also increased. Twice-daily injections of clenbuterol (2 mg/kg body weight/d) to fasting animals had no effect on BW or the mass or protein content of gastrocnemius muscle or liver, but both parameters were stimulated in heart. The results indicate that the anabolic action of clenbuterol are maintained when substrate availability is reduced by food restriction, but this effect is lost during severe protein and energy deficit (fasting).