The role of insulin as a possible mediator of the beta-adrenergic agonist stimulation of muscle growth was investigated. To exclude possible action of the beta-agonist on the pancreatic release of insulin, diabetes was induced in rats by a streptozotocin injection (100 mg/kg). Insulin levels were almost not detectable in these rats. Feeding either normal diet or diet containing the beta-adrenergic agonist clenbuterol (10 parts/million) did not alter plasma insulin concentrations. The effects of clenbuterol on muscle and weight gain were determined in diabetic rats given daily insulin replacement (D + I) and fed either a normal diet or clenbuterol-treated diet. Clenbuterol, fed for 1 wk, increased the wet weight of the gastrocnemius, soleus, and extensor digitorum longus muscles (15-23%) in both normal and D + I rats. Although clenbuterol increased body weight gain, it did not alter feed consumption and, therefore, feed efficiency (g gain/g food) was improved. Activities of cathepsin B and N-acetyl-beta-glucosaminidase, but not cathepsin D, were elevated in the soleus muscles of clenbuterol-treated rats. The clenbuterol-induced increase in muscle growth in the insulin-replaced diabetic rats indicated that this beta-adrenergic agonist effect was not mediated by an alteration of circulating levels of insulin, secondary to beta-agonist action on pancreatic insulin release.