A dose-response relationship between circulating gonadotropin levels and synthetic LRF has been established in 10 adult males for iv doses ranging from 1–450 μg. Though both serum LH and FSH concentrations increased with dose, the responses in FSH were much smaller and occurred later than those of LH. A statistically significant increase in LH was observed in response to the 10 μg dose and the mean responses increased from 1 to 150 μg. In contrast, a statistically significant increase in FSH response was observed only at the 150 μg and 450 μg doses. Maximum serum levels were observed at a median time of 25 min following LRF administration for LH as compared to 45 min for FSH. The great variability in response among subjects (which was not related to age or body surface area) demonstrates the importance of serial studies in the same individuals.