A method for perfusion of the rat ovary in vitro for the study of ovulation was developed and characterized. Immature rats (27-29 days old) were primed with 20 IU of pregnant mare's serum gonadotropin (PMSG). Two days later a laparotomy was performed, the aorta and vena cava were cannulated, the right ovary was isolated, and all vessels connecting with the aorta and vena cava except for the right ovarian artery and vein were ligated. The preparation was placed in the perfusion apparatus and perfused for up to 20 h with oxygenated Medium 199 containing 4% bovine serum albumin (37 degrees C, pH 7.4). The perfusion pressure was 70-90 mm Hg and the average flow was 1 ml . min . ovary. Ovulation was confirmed by oocyte recovery from the apparatus. Nine ovaries were perfused without further treatment (controls); only 1 ovulation occurred in this group. Nine ovaries received luteinizing hormone (LH; 0.1 microgram/ml) 1 h after the start of perfusion; 71 ovulations resulted (range, 3-13/ovary). Ovulations did not begin before 9 h after LH administration. Samples of medium were taken frequently for measurement of progesterone, estradiol and androstenedione. Levels of all three steroids rose rapidly and markedly in response to LH but increased only slightly in control perfusions. This study demonstrates that ovulation in the rat ovary can be induced in vitro and provides a basis for further studies on the mechanism of ovulation.