The present investigation has analysed certain aspects of the organization of the preoptico-tuberal system in relation to its role in activating the adenohypophyseal discharge of luteinizing hormone (LH). In Nembutal-blocked proestrous rats, transections were placed unilaterally in either the suprachiasmatic plane (SCH), the anterior hypothalamic area (AHA), the ventromedial-arcuate complex (VMN-ARC) or posterior hypothalamus. Whereas SCH or AHA transections failed to alter the basal concentrations of plasma LH, transections of the VMNAHC complex elevated plasma LH sufficiently to induce ovulation. In a second study the SCH region or AHA was transected and the medial preoptic area (MPOA) then was stimulated using electrical parameters previously shown to induce ovulation. When the transection was ipsilateral to the electrode, no alterations were observed in plasma LH and ovulation failed. On the contrary, MPOA electrical stimulation resulted in LH patterns and concentrations similar to controls when the transections were placed contralateral to the electrode. In two groups of chronically operated rats, the VMN-ARC complex was unilaterally transected or midsagittal cuts extending through basal hypothalamus from VMN-ARC to SCH were made. Following a recovery period, and with the return of vaginal cyclicity, Nembutal was administered on proestrus and the MPOA was electrically stimulated. All rats ovulated and patterns and concentrations of LH were within normal control ranges. Thus, we have been unable to demonstrate any significant crossover of preoptico- tuberal fibers anterior to VMN-ARC complex. (Endocrinology93: 369, 1973)