Ovulatory effects of histamine and specific antagonists were studied in isolated perfused ovaries from immature rats treated with 10 i.u. PMSG to stimulate follicular growth and maturation. Histamine alone, like LH, induced ovulation in all ovaries tested, but the number of follicular ruptures was lower after histamine (7.0 and 2.2 ruptures, respectively, per ovary). The histamine-induced ovulations could be inhibited dose-dependently by the H1-receptor antagonist, pyrilamine, or the H2-antagonists, cimetidine and ranitidine. At the concentrations tested, these antagonists did not, when given separately, reduce the LH-induced ovulations significantly, but pyrilamine and cimetidine in combination lowered the ovulation frequency by 65%. The prostaglandin synthesis inhibitor, indomethacin, was not able to block the histamine-induced ovulations.