Cervical stimulation was used to induce synchronous ovulation in rats that were induced into constant estrus by exposure to continuous lighting. Anti-LHRH or anti-LH serum completely blocked the induced ovulation when administered iv within 5 or 40 min after cervical stimulation. Peripheral concentrations of LH and LHRH hypothalamic content of LHRH were quantified before and at various intervals (0.5-70 min) after cervical stimulation. The radioimmunoassay for LHRH employed a specific antiserum, OAL-513, with high affinity and specificity and highly purified 125I-LHRH. Measurement of LHRH in extracts of 3-5 ml of plasma gave values that were readable in the middle portion of the standard curve. A two-fold increase in the LHRH concentration in peripheral plasma was observed 30 sec after the start of cervical stimulation. The elevated LHRH level was sustained for 15 min, though associated with large fluctuations. Serum LH levels started to rise 4 to 10 min and returned to the basal level 70 min after cervical stimulation. The hypothalamic LHRH content continued to increase throughout the entire experimental period and attained 140% of the initial level. No significant deflection due to LHRH release was observed in the hypothalamic LHRH content. Changes in the peripheral hormone level after cervical stimulation were component for chronologically reasoning the effectiveness of the antihormone treatment for blockage of the induced ovulation. From these results it is concluded that LHRH release before the preovulatory LH surge in light-estrous rats was brief in duration and erratic in mode.