The effect of di-n-propylacetic acid (valproic acid), a gamma-aminobutyric acid transaminase inhibitor, on the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) response to gonadotropin-releasing hormone (LHRH) was studied in five normal women during the proliferative and luteal phases of the menstrual cycle. Valproic acid produced no significant change in the basal serum concentrations of LH, FSH, estradiol, and progesterone in either the proliferative or the luteal phase of the study. In the proliferative phase the delta LH (maximum increment above baseline) following LHRH stimulation rose from 32.8 +/- 21.2 (mean +/- SD) to 52.2 +/- 28.7 mlU/ml (not significant) after valproic acid, while the delta FSH rose from 2.2 +/- 1.1 to 5.0 +/- 3.6 mlU/ml (not significant). Four of the five volunteers showed an augmentation of the delta LH response to LHRH after valproic acid while the fifth subject showed no change. In three subjects the augmented delta LH response after valproic acid was highly significant. By contrast, the delta LH in the luteal phase following LHRH stimulation fell from 65.3 +/- 20.1 to 43.1 +/- 12.9 mlU/ml after valproic acid (p less than 0.03). Corresponding delta FSH values were 2.5 +/- 1.1 and 2.1 +/- 0.8 mlU/ml (not significant). It is speculated that gamma-aminobutyric acid may exert a modulatory role on gonadotropin secretion following LHRH stimulation and that this effect is influenced by the phase of the menstrual cycle.