Regulating the time of ovulation in the mare has many practical and beneficial management applications, including use with artificial insemination. The objective of the present study was to test the effectiveness of Cystorelin(TM) (gonadorelin diacetate tetrahydrate) to elicit increased LH secretion and to induce ovulation in cyclic mares. A total of 24 mares of Thoroughbred and stock type breeding was used in this study. Mares possessing an ovarian follicle 3.5 cm to 4.0 cm in diameter were assigned to either treatment with three 75 microg (1.5 mL) i.m. injections of Cystorelin(TM) or three 1.5 mL i.m. injections of sterile saline (control). Treatment with Cystorelin(TM) reduced (P<.05) the mean number of days until ovulation (2.25 +/- 0.25 vs 3.23 +/- 0.41 for Cystorelin(TM) and saline, respectively), although variation in the time to ovulation was similar (P>.81). Mean plasma LH levels were calculated over four time periods (-30 to 0 (Period 1), 30-120 (Period 2), 150-240 (Period 3), 270-360 min (Period 4)). Luteinizing hormone response was analyzed by comparing the mean LH value of Period 1, 2, and 3 with the baseline mean. Although the level in Period 2 tended (P<.07) to be higher than baseline, LH levels in Periods 3 and 4 were not different from baseline (P>.15, P>.18 respectively). In summary Cystorelin(TM) effectively hastened ovulation in mares. Further research is necessary to develop a protocol that can also reduce variability in the time of ovulation.