Six pony mares were ovariectomized (OVX) on day 16 of diestrus during June and July, 1972, to study short term changes in plasma luteinizing hormone (LH) concentrations. Plasma LH was higher (P less than .05) 3 days after OVX (1.76 ng/ml) than the day after OVX (1.01 ng/ml), and a gradual increase occurred over the first 2 weeks. Elevated plasma LH concentrations similar to mid-estrus levels were present from the 2nd to 11th week post-OVX. In another experiment, the same 6 OVX mares were bled once a month from February, 1973, to January, 1974, to study long-term changes in plasma LH in relation to the anestrous (November-April) and breeding seasons (May-October). Mean monthly plasma LH concentrations in OVX mares during all months of the anestrous season were lower (P less than .05) than mean monthly plasma LH during the months of June-September of the breeding season. During the transition from anestrous to breeding seasons (April, May) and breeding to anestrous seasons October, November), mean plasma LH concentrations were not significantly different between April and May (1.39 vs 1.84 ng/ml) and between October and November (3.02 vs 2.56 ng/ml), but April and May levels were lower (P less than .05) than October and November levels. Mean monthly plasma LH levels in OVX mares during the anestrous season were similar to those in intact mares during the anestrous season and at mid-diestrus during the breeding season. Mean monthly plasma LH levels in OVX mares during the breeding season were similar to plasma LH levels at mild-estrus in intact mares. These data indicate the presence of inhibitory (most likely progesterone) and stimulatory (most likely estrogen) influences from the ovary which modulate the endogenous LH pattern in mares during the breeding season.