Hypothalamic content of LHRH was radioimmunoassayed after cervical stimulation on the morning of day of estrus' LHRH content rose 3-fold by 120 min after the stimulation, reaching a level equivalent to that of day 5 of pseudopregnancy. The increase was not steady as there were two early periods when the content did not increase. Measurement of pituitary LH content revealed a marked increase followed by a sudden decrease which corresponded respectively to the two periods when LHRH levels failed ro tise. These changes in pituitary LH level suggest that both the synthesis and release of LH in the pituitary are promoted by the action of LHRH released from the hypothalamus. LH changes in the pituitary were abolished iv injection of 2 ml LHRH antiserum at the time of cervical stimulation, but pseudopregnancy was still induced and diestrus prolonged 4 days longer than in the control group. Progestin concentrations in ovarian vein blood were comparable between anti-LHRH-treated and control groups. These results suggest that the LH released by cervical stimulation is dispensable for the induction of pseudopregnancy. The marked elevation of LHRH in the hypothalamus observed immediately after cervical stimulation must be related to the regulatory mechanism of the characteristic secretory pattern of LH during the period of pseudopregnancy.