Pituitary 5α-reductase activity in intact male rats increases after the pituitary is incubated with the hypothalamus. Incubating the pituitary of castrate rats with the hypothalamus of intact rats relatively inhibits pituitary 5α-reductase activity. Coincubation of the pituitary and hypothalamus of castrate rats, or the pituitary of intact with the hypothalamus of castrate males, does not elicit changes in pituitary 5α-reductase activity. Different amounts of LH-RH in the incubation medium can modify 5α-reductase activity, i.e., activate it in the intact pituitary and inhibit it in the castrate pituitary. Hypothalamus from intact rats, which according to Shin et al. [1974] is ‘rich’ in LH-RH, induces changes in pituitary 5α-reductase activity. The LH-RH-’poor’ hypothalamus of castrate rats does not cause changes in this enzyme activity. The results suggest that there is a very close relationship between LH-RH and 5α-reductase activity in