Previous work by our laboratory demonstrated that activation of the progesterone receptor through exogenous administration of progesterone suppressed glutamic acid decarboxylase-67 (GAD<sub>67</sub>) mRNA in the hypothalamus of the estrogen-primed ovariectomized rat. Since GAD<sub>67</sub> is the major synthetic enzyme for the inhibitory transmitter, γ-aminobutyric acid, the finding raised the possibility that the endogenous activation of the progesterone receptor may act to restrain GAD<sub>67</sub> expression during the natural preovulatory gonadotropin surge during proestrus in the rat, thereby allowing GnRH secretion and the resultant LH surge. To test this hypothesis, the progesterone receptor antagonist, RU486, was administered to regularly cycling proestrous rats and the effect on GAD<sub>67</sub> and GAD<sub>65</sub> mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH) was examined. Serum luteinizing hormone (LH) levels were also examined in order to identify correlations between changes in POA and MBH GAD levels and production of the LH surge. GAD<sub>67</sub> mRNA levels in the POA were increased in the cycling rat during proestrus at 18.00 h at the peak and just preceding the termination of the LH surge. There was no change in GAD<sub>67</sub> mRNA levels in the MBH, and GAD<sub>65</sub> expression was also unchanged during proestrus in the POA and MBH. Treatment with the antiprogestin RU486 resulted in an increase in GAD<sub>67</sub> mRNA levels at 12.00 and 14.00 h in the POA, and in the MBH at 14.00, 16.00, and 18.00 h during proestrus, effects which preceded and correlated with the attenuated LH surge in RU486-treated rats at 18.00 h. GAD<sub>65</sub> mRNA levels were also elevated by RU486 at 14.00 and 16.00 h in the POA, and at 14.00 h in the MBH during proestrus. These findings suggest that the progesterone receptor plays a role in restraining GAD expression in the hypothalamus during proestrus, and that this effect may be important for the production of the GnRH and LH surge.