The effects of the (+)- and (-)-isomers of propranolol and INPEA on the neuromuscular blocking action of (+)-tubocurarine and succinylcholine were studied in the tibialis anterior-peroneal nerve preparation of the rabbit and the isolated phrenic nerve-diaphragm preparation of the rat. (+)-Tubocurarine-induced neuromuscular blockade was intensified by (+)- and (-)-isomers of INPEA whereas it was unaffected by isomers of propranolol in both preparations. In the rabbit tibialis anterior-peroneal nerve preparation, only the (-)-isomers of propranolol and INPEA increased the duration of succinylcholine-induced neuromuscular blockade without affecting its magnitude. Higher doses of both (+)- and (-)-isomers of propranolol and INPEA potentiated succinylcholine-induced blockade in the rat phrenic nerve-diaphragm preparation. The results obtained indicate that only the increase in duration of succinylcholine-induced blockade with beta-adrenoceptor blockers can be related to beta-blocking property of these compounds.