The rhythmical activity of the liver fluke, Fasciola hepatica, was stimulated by 5‐hydroxytryptamine and by lysergic acid diethylamide at very low concentrations. The effect was peripheral and was not mediated through the central ganglion. Other amines also stimulated rhythmical activity, the most potent being the indolamines.

Bromolysergic acid diethylamide, and other analogues such as yohimbine, harmine, and dopamine, depressed rhythmical movement and antagonized the stimulant action of 5‐hydroxytryptamine and lysergic acid diethylamide. Evidence which suggests the presence of tryptamine receptors in the trematode is discussed.