Field stimulation was used to elicit a contractile response in muscle strips from rabbit detrusor. The blockade of this response by tetrodotoxin (1 .times. 10-7 M) ranged from 100% at 1 Hz to 86% at 40 Hz. At concentrations which produced strictly muscarinic antagonism (up to 4 .times. 10-7 M) atropine depressed the frequency-response curve by about 42% at maximum but was much less effective at frequencies below 10 Hz. Treatment of the strips with hemicholinium-3 (5.2 .times. 10-4 M) for 90 min in the presence of field stimulation at 60 Hz, depressed the frequency-response curve by 52% at maximum but produced less depression below 10 Hz. The hemicholinium-3-resistant response was neither depressed further by atropine (4 .times. 10-7 M) nor potentiated by physostigmine (2 .times. 10-6 M). Although hemicholinium-3 had antimuscarinic and anticholinesterase properties, these did not interfere with the tests for residual cholinergic transmission. Apparently only part of the motor neurotransmission in rabbit detrusor is cholinergic. The remaining portion is predominant at frequencies below 10 Hz and is postulated to involve a chemical mediator other than acetylcholine.