Primarily on the basis of indirect observations, workers have speculated that endogenous release of LH in immature rats occurs between 2 and 6 p.m. on the 2nd day, i.e., at 53–57 h after PMS injection. Utilizing a specific radioimmunoassay for rat LH, we have been able to show that this release, although variable, usually occurs between 54 and 57 h after the administration of PMS. Furthermore, we have shown that interruption of all neural tracts to and from the medial-basal hypothalamus and, more specifically, interruption of those entering and exiting anteriorly to the medial-basal hypothalamus prevents the elevation of plasma LH levels at the usual times and subsequently ovulation does not occur. In general, these observations confirm those seen in the adult female rat.