Serotonin (5-hydroxytryptamine, 5-HT) receptors in the muscle and intestinal tissues of adult Ascaris suum have been investigated. [3H] lysergic acid diethylamide (LSD) exhibited specific and saturable binding to membranes prepared from both intestine and muscle. The intestinal tissue membranes had an equilibrium dissociation constant (Kd) of 2.70 nM for LSD and a Kd of 2.50 microM for 5-HT. As compared to the intestine, the muscle membranes had comparatively higher affinity for both LSD (Kd = 1.80 nM) and 5-HT (Kd = 0.68 microM). The muscle membranes also had a high binding affinity for ketanserin, a 5-HT2 antagonist, (Kd = 16.7 nM) whereas intestinal membranes exhibited no specific binding of ketanserin. Serotonin significantly inhibited the binding of LSD to the intestinal and muscle tissue membranes while adrenergic and cholinergic drugs and histamine did not. This suggested that the binding of LSD, 5-HT and ketanserin to the parasite membranes was specific. Collectively, the data demonstrated the presence of a serotonin receptor in the muscle and intestinal tissues of the adult A. suum. The receptor in the muscle was pharmacologically similar to the mammalian serotonin type 2 receptor.