Ascaris suum possesses a well-developed nervous system which is regulated by a number of classical neurotransmitters including acetylcholine (ACh), γ-aminobutyric acid (GABA), glutamate and serotonin. The vagina vera, the distal part of the ovijector, displays intrinsic, rhythmic activity which has been shown to be modulated by FMRFamide-related peptides (FaRPs) in vitro. Confocal scanning laser microscopy coupled with immunocytochemistry, and histochemical studies, revealed that the nerve plexus of the ovijector contains GABAergic and glutamatergic innervation. Although no distinctive cholinergic or serotoninergic innervation was apparent, cholinesterase activity was localized to discrete areas of the musculature of the vagina vera. The effects of classical transmitters on the activity of the vagina vera in vitro were examined. ACh was excitatory, stimulating a brief but powerful contraction of the vagina vera with a threshold for activity of 1 μM. Both GABA and glutamate were inhibitory, causing a cessation of contractile activity at high concentrations (> 10 μM). Although less potent than glutamate, GABA had more profound effects and induced longer-lasting paralysis of the tissue. The threshold concentrations for activity were 5 μM for glutamate and 10 μM for GABA. Serotonin had no consistent effect on the vagina vera. This study demonstrates that classical transmitters modulate the activity of the ovijector of A. suum.