THIS study was performed to test the hypothesis that diminution in the preovulatory luteinizing hormone (LH) surge in middle-aged (MA) rats may be related to altered activity in nitric oxide (NO)ergic neurons. To begin to test the hypothesis, gene expression of hypothalamic NO synthase (NOS), the enzyme that catalyzes NO production, was examined on the day of proestrus in young (2–3 months old) and middle-aged (MA, 8–10 months old) rats. Compared with those at 10.00 h, NOS mRNA levels were increased significantly at 12.00 h and the LH surge occurred at 16.00 h in young rats. In MA rats, in contrast, there was no change in NOS mRNA levels between morning and afternoon hours in association with a delayed and attenuated LH surge. These results show that hypothalamic NOS gene expression increases long before the LH surge in young rats and that this antecedent increase in NOS gene expression does not occur in MA regularly cycling rats. Since hypothalamic NOergic neurons participate in induction of LH-releasing hormone (LHRH) and LH surges, our results suggest that reduced LH surges in MA rats may, in part, be due to altered NOS neuronal activity in these animals.