The pituitary response to LH-RH was studied at 8 a.m.,4 p.m. and 12 p.m. in normal male rats, female rats on day 1 of diestrus and female rats injected with testosterone propionate on the 2nd day of life. Androgenized rats responded to LH-RH with a greater release of LH than did female diestrous and male rats. In the 3 groups the lowest response to LH-RH was observed at 12 p.m., and the highest at 8 a.m., with the exception of androgenized rats which responded with a similar magnitude of LH release at 8 a.m. and 4 p.m. The release of FSH after the injection of LH-RH was studied only at 4 p.m. and only normal male rats responded with a significant FSH release.