Serum estradiol (E2), luteinizing hormone (LH), and follicle stimulating hormone (FSH) levels were measured in blood samples obtained prior to and at frequent intervals for 360 or 480 minutes following the subcutaneous administration of gonadotropin releasing hormone (Gn-RH) to eight normal women and nine patients with hypothalamic secondary amenorrhea. In the normal subjects given 100 mug of Gn-RH on day 7 of the menstrual cycle, there was no significant increase in the mean E2 concentration above basal levels. Six women with secondary amenorrhea received Gn-RH, 100 mug, for 4 successive days; frequent blood samples were obtained on days 1 and 4. On day 1, the mean E2 concentration at 360 minutes was significantly greater than the mean basal level. On day 4 no significant increase in E2 was detected, although the mean LH and FSH responses and basal E2 level were not different from those of day 1. Four women with secondary amenorrhea received daily doses of 500 mug of Gn-RH for 7 days. Significant increases in mean serum E2 concentration (100 to 150 pg/ml) were noted at 6 and 8 hours after administration on day 1 and at 8 hours on day 4. No significant rise in E2 was detected on day 7. The mean LH and FSH responses did not differ from those observed in women who received the 100-mug dose, and there were no significant differences in responses on successive days of Gn-RH administration.