1 In the pithed rat, muscarine (2.5-10 microgram/kg i.v.) normally produced bradycardias, but tachycardias were seen in the presence of pancuronium (0.1-1.0 mg/kg i.v.) and alcuronium (0.1-5.0 mg/kg i.v.). 2 The tachycardia was probably a result of muscarinic receptor stimulation, because it was antagonized by atropine (10 microgram/kg i.v.) and pirenzepine (10-30 microgram/kg i.v.). The location of these muscarinic receptors is probably within the sympathetic ganglia since the tachycardias were inhibited by pretreatment with propranolol (1 mg/kg i.v.) or reserpine (5 mg/kg i.p. 24 h prior to study). 3 In the rat isolated atria preparation, pancuronium was 86 times more potent as an antagonist of the effects of muscarine than in the rat isolated superior cervical ganglion. The mechanism of action in both tissues may be non-competitive. 4 Pancuronium selectivity antagonized atrial inhibitory muscarinic responses compared to excitatory muscarinic responses in sympathetic ganglia in the rat.