S-[3-(N,N-Dimethylamino)propyl]isothiourea (dimaprit), has been shown to be a highly specific histamine H2-receptor agonist. Parallel line assays showed that in vitro at H2-receptors it had approximately 17.5% the activity of histamine on the rat uterus and 71% on the guinea-pig right atrium, with similar maximal responses; it had less than 0.0001% the activity of histamine on H1-receptors. Dimaprit stimulated gastric acid secretion in the rat, dog and cat in which it had, respectively, approximately 19,58 and 400-500% the activity of histamine. In the dog and cat the maximum secretory response to dimaprit was significantly greater than that obtained to histamine. The H2-receptor specificity of dimaprit in causing depressor and vasodilator effects was also demonstrated in the cat, in which it had 18-20% of the H2-receptor activity of histamine. Dimaprit should prove to be a very useful tool in studies examining the role of histamine in physiology and pathology. The absence of marked cardiovascular effects at doses maximal for the stimulation of gastric acid secretion, as seen in the cat studies, could lead to this compound being of value as a diagnostic agent in the measurement of maximal acid secretory capacity.