In the present studies we have simultaneously measured changes in medial basal hypothalamic (MBH) leutenizing hormone-releasing hormone (LH-RH) and in plasma LH by radioimmunoassay in female rats at various hours during the 4-day estrous cycle and under experimental conditions known to alter pituitary LH secretion. In groups of rats decapitated at 12.00 h and 15.00 h on estrus and diestrus, plasma LH remained at basal levels (5-8 ng/ml) and MBH-LH-RH concentrations showed average steady state concentrations of 2231 +/- 205 pg/mg. On the day of proestrus hourly measurements of MBH-LH-RH between 12.00 h and 21.00 h suggested rhythmic rises and falls in the decapeptide concomitant with rises and falls in plasma LH. In a second group of pentobarbital-anesthetized proestrous rats a significant decline in MBH-LH-RH occurred (to 573 +/- 137 pg/mg) which then remained at low concentrations between 14.00 h and 18.00 h proestrus. Following bilateral preoptic area (MPOA) electrochemical stimulation of pentobarbital-treated proestrous rats, LH was significantly increased by 30 min, peaked between 90-120 min and returned to basal levels by 210 min poststimulation. In the same animals within 15 min poststimulation, MBH-LH-RH increased from the basal concentrations noted after pentobarbital anesthesia to elevated levels comparable to those observed throughout estrus, diestrus and on proestrous morning. Further, as plasma LH rose to peak concentrations and declined to basal plasma values, rhythmic rises and falls in MBH-LH-RH were observed with intervals between pulses of approximately 60 min. Seemingly, hypothalamic LH-RH is released as pulsatile pulses from a releasable pool; this pool is replenished and again LH-RH is discharged in response to constant stimulation by the preoptic brain.