The rat pars tuberalis was studied with conventional electron microscopy and immunocytochemistry for the demonstration of luteinizing hormone (LH). The LH-secreting cells were preferentially located in two regions of the pars tuberalis (PT), namely, that surrounding the neural stalk and that occupying the tuberoinfundibular sulci. Dialyzed extracts of PT prepared after removal of the pituitary stalk, had the capacity to induce ovulation in chlorpromazine-blocked rats in proestrus. In radioimmunoassays carried out under the same conditions, the PT extracts yielded displacement curves parallel to those of standard LH. The immunoreactive LH content of the female PT was determined at three phases of the estrous cycle: diestrus, afternoon of proestrus and estrus. The lowest values were found in rats sacrificed in the evening of proestrus (18.00 h), and they were about one sixth of the peak values found at estrus. 1 month after inducing lesions in the preoptic area of female rats, the LH content of the PT was four times higher than the highest values found during the estrous cycle. The results suggest that the rat PT does secrete LH, and that this secretory activity fluctuates with the estrous cycle, but in a manner that differs from that reported for LH secretion of the pars distalis.