A gonadotropin-releasing factor could be extracted from human hypothalamic tissue. Follicle-stimulating hormone (FSH)- and luteinizing hormone (LH)-releasing activities were assayed according to Ramirez and McCann. FSH and LH were testet according to Steelman and Pohley or Parlow. A minimal FSH-releasing activity was caused by 0.01 μg human FSH-releasing factor. The optimal dose was achieved with 0.025 μg. An increase of the dose up to 0.3 μg did not result in a further FSH release. 0.01 μg of the synthetic LH-/FSH-releasing factor showed a significant FSH release. An increase of the dose did not result in an increased FSH release. In contrast, the LH release was dose-dependent. The course of FSH release after application of 0.1 μg human gonadotropin-releasing factor showed the maximum after 30 min and reached the normal level after 2 h. Injection of 0.01 μg synthetic LH-releasing factor produced a maximum after 10 min and a smaller maximum after 60 min. The course of LH release after injection of 0.2 μg human gonadotropin-releasing factor showed a peak maximum 30 min and a second maximum 120 min after application. The synthetic LH-releasing factor showed a peak maximum after 5 min, followed by declines and maximas between 30 and 45 min and 100–120 min. Human gonadotropin-releasing factor and synthetic LH-/FSH-releasing factor showed a different behaviour. Chemical differences may be responsible for these differences.