Growth hormone (GH) supplementation can increase the body weight of old rats, but the individual tissues affected were previously unidentified. Therefore, the masses of the heart, spleen, kidney, epididymal fat pads, and five skeletal muscles were assessed in male Fischer 344/Brown Norway rats (9, 20, 31, months) injected with recombinant human GH (0.7 mg/kg) or vehicle twice daily for 10 days. Muscle composition (fiber type, protein concentration, dry weight/wet weight ratio, citrate synthase activity) was also evaluated. Muscle mass was increased with GH treatment, and this increment was undiminished in old age. Fiber type, protein concentration, and dry weight/wet weight ratio were unaffected by GH. Citrate synthase activity declined in the plantaris and increased in the soleus with GH treatment. GH supplementation elevated heart and spleen mass, but not fat pad or kidney weight. The data demonstrate that the capacity for GH-induced hypertrophy of skeletal muscle, myocardium, and spleen is retained during old age.