Ovaries from sexually mature virgin rabbits were dissected free on both sides and cannulated in situ via that part of the aortic segment supplying the ovarian arteries. The ovaries were mounted in a closed, recirculating perfusion system for oxygenation in vitro. Ovulation was induced by human chorionic gonadotropin (hCG) given either i.v. (100 IU) or directly into the perfusion medium (20 IU). Follicular ruptures occurred in 42 out of a total of 54 ovaries. The in vivo route of hCG administration was found to be most efficient, with ovulations occurring after a mean of 11.5 h, which corresponds well with the incidence of ovulations in situ. The prostaglandin synthesis inhibitor, indomethacin, totally abolished the hCG-induced ovulations. beta-Adrenoreceptor stimulation in vitro with terbutaline or with noradrenaline in the presence of alpha-receptor blockade with phenoxybenzamine caused a significantly increased number of gonadotropin-induced ovulations. Phentolamine or phenoxybenzamine given alone reduced the number of ruptured follicles. Noradrenaline (in the absence of hCG) and clonidine augmented the incidence of ovulation. The results indicate that local adrenoreceptor mechanisms participate in the process of ovulation, both via alpha-adrenoreceptors (probably affecting the follicular smooth musculature) and beta-adrenoreceptors (possibly through some trophic or endocrine mechanism other than a mechanical factor).