Diallylmelamine was tested in man as a gastric antisecretory agent. In these tests it was inactive. However, when this compound was administered to trained, unanesthetized dogs, it was surprisingly found to have pronounced and prolonged hypotensive activity. Diallylmelamine is an effective hypotensive agent in hypertensive dogs and rats, having a duration of action exceeding twenty-four hours from a single oral dose. It has limited efficacy in normotensive rats. Hypotensive activity of gradual onset is preceded by a latent period of up to two hours and becomes maximal six hours or more after dosing. This agent does not depress cardiac output or sympathetic vasoconstrictor activity.