Out of 11 agonists, including such agents as bradykinin, prostaglandin E2, eledoisin, only acetylcholine (Ach) caused the isolated exposed ascaris muscle to contract during treatment for 30 sec and at 10(-7) g/ml (or lower concentrations). The possibility of using this preparation for the identification and quantification of Ach in biological tissues and fluids was highlighted. Neostigmine, but not physostigmine, augmented Ach activity. This suggests that cholinesterases present in the ascaris muscle may be structurally dissimilar to those in the mammalian gut muscle. Several chemically unrelated antagonists showed anti-Ach activity in this preparation in a descending order: d-tubocurarine (dtc) greater than mepyramine greater than atropine, decamethonium, succinylcholine greater than hexamethonium greater than piperazine. Neither adrenaline nor noradrenaline relaxed the ascaris muscle but gamma-aminobutyric acid (GABA) and, occasionally, piperazine caused immediate relaxation of the muscle. GABA was about 150 times more potent than piperazine in this regard. GABA also showed anti-Ach activity which was 0.8-3 times that of dtc. It is suggested that GABA is the inhibitory transmitter in the ascaris suum and that the anti-Ach activity of piperazine is due either to stimulation of GABA receptors and/or to nonspecific blockade of Ach receptors.