Seven-day-old primary myotube cultures derived from embryonic chicken limb muscles were used to determine the effects of the beta-adrenergic agonist isoproterenol (ISO) on muscle protein metabolism in vitro. Isoproterenol increased (P less than .05) total protein accumulation after 2 h of acute exposure and after chronic exposure for 24 and 48 h. Isoproterenol did not consistently retard rate of protein degradation of the total protein (TP), myofibrillar protein (MFP) pools, and myosin heavy-chain subunit (MHC); degradation of these protein pools tended to be slowed by inclusion of ISO in the culture medium. After acute treatment of 1 X 10(-4) M ISO for 2 h, TP, but not MFP and MHC, synthesis rate was increased, and after chronic exposure to 1 X 10(-4), 1 X 10(-5), and 1 X 10(-6) M ISO, TP, MFP, and MHC synthesis rates and net accumulation of TP, cytoplasmic protein, and MHC fractions were enhanced (P less than .05). The beta-adrenergic antagonist propranolol (1 X 10(-5) M) blocked chronic stimulatory effects of ISO. Furthermore, after 48 h of exposure to ISO, effects on protein synthesis were less pronounced than those observed after 24 h of exposure. Isoproterenol imparted a more pronounced effect on protein synthesis than on protein degradation, indicating that increased muscle protein accretion observed in animals after ISO treatment is likely a function of enhanced protein synthesis.