A previous study has shown that chronic chemical sympathectomy brought about by 6-hydroxydopamine injections results in a dpression in myocardial contractile function which is accompanied by reduced myofibrillar ATPase activity. To determine whether chronic beta-adrenergic receptor blockade elicits similar alterations in cardiac contractile-protein ATPase activity, adult rats were given twice-daily injections of propranolol 7 days/wk for 2 wk. Effective beta-adrenergic receptor blockade was verified by the lack of hemodynamic responsiveness to isoproterenol infusion. Myofibrils were prepared from left ventricular tissue and analyzed for ATPase activity. Myofibrillar ATPase activity was 295 +/- 8 nmol Pi.mg-1.min-1 in controls. Enzyme activity was not significantly different in propranolol-injected rats. The results demonstrate that chronic propranolol administration does not alter the ATPase activity of cardiac myofibrils. Therefore, it seems likely that the altered contractile-protein enzymatic properties resulting from chronic chemical sympathectomy do not occur as the result of a reducted level of cardiac beta-adrenergic receptor stimulation.