The effects of histamine infusions on the rat uterine vasculature have been studied using the radioactive microsphere technique. Histamine produced a marked uterine vasodilatation which could be partly mimicked by infusion of either the H 1-receptor agonist, 2-(2-aminoethyl)pyridine, or the H 2-receptor agonist, 4-methylhistamine. Histamine-induced uterine vasodilatation could only be reduced significantly by administration of a combination of the H 1- and H 2-receptor antagonists mepyramine and metiamide but not by either antagonist given alone. It was concluded that histamine-induced vasodilatation in the rat uterus is mediated by both H 1- and H 2-receptors.