In a second set of experiments, the time course of LU, FSH, estradiol (E₂) and progesterone (P₄) was investigated, between proestrous afternoon (1630 h) and estrous morning (1000 h), in 3 treatment groups: a) saline controls (no PB), b) PB controls (no LH), c) PB + LH (8 µg IV at 1530 h). a) Saline controls showed a maximum LU and FSU primary surge at 1830 h; LH returned to baseline by 2300 h, while FSH fell slowly through the night. P₄ was elevated at 1830 hand 2300 h, then fell to baseline; E₂ fell abruptly between 1630 h and 1830 h. b) In PB controls, serum LH and P were continuously low, but a small rise in FSH occurred at 2300 h and 0400 h (estrus). E₂ fell significantly also, but not as low as in the saline controls. c) LH, superimposed on PB, raised LH at 1630 h, but no further rises occurred, while FSH rose gradually to peak at 0400 h and then fell part way to saline control levels by 1000 h estrus. Thus, FSH appeared to be released independently of LU. The injected LU also induced a rise in P₄ at 1630 h and 1830 h, with E₂ falling, as in saline controls, between 1630 h and 1830 h. In a third experiment, P₄ injection was tested, superimposed on PB blockade. P₄ failed to increase LH or FSH (in a manner resembling LH injection), but did cause E₂ to fall. It is concluded that the primary LH (and/or FSH) surge, as well as exogenous LH or FSH superimposed on PB blockade, can induce a release of FSH independently of changes in measured steroids (serum P₄, E₂) and in the absence of concomitant LH release. The induction of FSH release could not be detected in the absence of the ovary, but ovariectomy by itself increased FSH release without LH release. The data suggest that gonadotrophins, as well as ovariectomy, may induce FSH release by altering a nonsteroidal feedback from the ovaries.