Various supra- and retrochiasmatic cuts injuring different parts of the septo-preoptico-infundibular GnRH pathway were made in adult female rats, and their effects on ovulation, median eminence (ME) GnRH content and on plasma LH and FSH levels were studied. Extended retrochiasmatic frontal cut just behind the optic chiasm, or a frontal cut in front of the suprachiasmatic nucleus presumably interrupting the whole GnRH pathway blocked ovulation, led to persistent estrus with polyfollicular ovaries, and reduced the ME GnRH content to 10 and 32%, respectively, expressed in percentage of unoperated control value. Severance of the GnRH pathway on one side or partial interruption of the pathway on the two sides in the retrochiasmatic area did not interfere with ovulation, and the ME GnRH content was 50% or more of the control value. Disconnection of the GnRH fibers coming from the septum resulted in a more than 30% decrease in the GnRH content of the ME, but did not block ovulation. Two separate symmetrical lateral cuts in the suprachiasmatic area leaving intact the presently known GnRH pathway reduced the ME GnRH content to 40% of the intact value, but did not interfere with ovulation. Plasma LH and FSH levels of the animals with different knife cuts were considerably dissociated and there were no apparent correlations between brain interventions and the concentrations of these two hormones in the blood. Our previous findings together with the present data suggest that: (1) half of the GnRH pathway, medial or lateral bundles of the pathway on the two sides are sufficient for ovulation and cyclic gonadotrophic function; (2) persistent estrus develops if the ME GnRH content is below 40% of the control value, and (3) about 60% of the ME GnRH originates outside the preoptic, supra- and retrochiasmatic region. Half of this 60 % may come from the septum and the vertical part of the diagonal band of Broca, the other half from the region in front of the preoptic area. The remaining 40% presumably arises from the preoptic (7–10%), supra- (15–20%) and retrochiasmatic region (8