Ovarian follicular cysts of cattle were defined as follicular structures (.gtoreq. 2.5 cm, diameter) which persisted for 10 days or longer in the absence of functional luteal tissue. Thirty dairy cows with ovarian follicular cysts were allotted to 6 groups (5 cows/group) and each was given 0 (saline solution), 25, 50, 100, 150 or 250 .mu.g of gonadotropin-releasing hormone (GnRH) by i.m. injection. Samples of blood were collected before GnRH was injected (0 h, at 0.25, 0.50, 0.75, 1, 2, 3 and 4 h, and at 1, 7, 11, 15 and 20 days after treatment. Five additional cows with follicular cysts were treated i.m. with 10,000 units of human chorionic gonadotropin (HCG), and blood sample collections were made before treatment (0 h) and on days 1, 7, 11, 15 and 20 after treatment. Serum luteininzing hormone (LH) concentration was not altered in cows given saline solution, but was increased significantly in cows given any of the doses of GnRH (in a dose-related manner). Peak LH responses occurred about 2 h after GnRH was given, and by 4 h LH was beginning to decrease. Serum progesterone concentrations increased by more than 2.0 ng/ml by day 11 after treatment in 18 of 20 cows treated with 50, 100, 150 or 250 .mu.g of GnRH. Progesterone responses in these cows were greater (P < 0.05) than in cows given saline solution or a 25-.mu.g dose of GnRH. Mean progesterone response to the 4 large doses of GnRH was similar in magnitude and duration to serum progesterone changes during the luteal phase of the bovine estrous cycle. After cows were treated with HCG, serum progesterone values were similar to those in cows given GnRH (50-250 .mu.g). In 19 cows, irrespective of treatment (0-250 .mu.g of GnRH), in which serum progesterone increased by more than 2.0 ng/ml by day 11 after treatment, serum estradiol, estrone and LH concentrations decreased (P < 0.01) over the same interval. Serum estradiol, estrone and LH remained at pretreatment concentrations in 9 cows in which progesterone did not change over the 11-day period. Estrus was observed within 30 days of treatment in 11 of 20 cows given 50, 100, 150 or 250 .mu.g of GnRH and probably occurred in 5 additional cows (no signs) according to serum progesterone changes. Of 20 cows, 16 seemed to come in estrus within 30 days of treatment; 4 of 5 HCG-treated cows were in estrus within 30 days of treatment. Two few cows were bred after these treatments to make meaningful observations on fertility. Single i.m. injections of GnRH were capable of consistently causing elevated serum LH with subsequent formation of luteal tissue in cows with ovarian follicular cysts.