In order to elucidate neural pathways concerned with the proestrous surge of LH, FSH and prolactin (Prl) release, brain transection or lesion was made acutely under ether anesthesia between 12.00 and 14.00 h of proestrus, and electrochemical stimulation was done under anesthesia with pentobarbital sodium (31.5 mg/kg b.w.) injected at 13.45 h. Transection which interrupted the connection of septum (SEPT), diagonal band of Broca (DBB) and bed nucleus of stria terminalis (BST) with the preoptic-suprachiasmatic area interfered with ovulation and surge of release of all 3 hormones. Isolation of the basal part of the suprachiasmatic area, including the suprachiasmatic nucleus (SCH), blocked ovulation also. Bilateral lesions in the medial preoptic area (MPO) with platinium-iridium electrode blocked ovulation and the surge of LH and Prl release, but not of FSH. Lesions in the SCH blocked ovulation and the surge of LH, but not of FSH and Prl. In the rat with acute isolation of the basal part of the suprachiasmatic area and SCH, stimulation of the MPO failed to induce ovulation and LH release, but was followed by FSH release. Prl release was not inhibited as in the intact rat. When the rat had the antero-SCH cut, stimulation of the SCH induced LH release but not FSH, and the inhibition on Prl release was pronounced. These findings offer evidence that the limbic-forebrain inputs are necessary for the preoptic integration in order to stimulate the proestrous surge of LH, FSH and Prl release. Furthermore, it is possible that separate pathways from the preoptic area to the medial basal hypothalamus are concerned in the stimulation of individual hormones -a restricted route for LH which may pass through the SCH, a diffuse one for FSH which may pass through either the SCH or anterior hypothalamic area, and a relatively diffuse one for Prl which may pass outside the SCH.