The preovulatory surge of LH is thought to be governed in rats by a diffuse neural pathway from the medial preoptic area (mPOA) to the medial basal tuber. Previous indirect evidence indicated that when part of this system is activated by electrochemical stimulation the amount of LH released depends on the span of the irritative lesion. The present investigation contributes substantiating evidence by radioimmunoassay of serum LH. In both the Osborne- Mendel (O-M) and Charles River CD strains, 4-day cyclic rats in proestrus and 5-day cyclic rats on diestrus day 3 (D-3), under pentobarbital during the midafternoon, were subjected to large or small unilateral stimuli from continuous anodic current through stainless steel electrodes. In the O-M rats, sequential 0.5 ml blood samples were taken via iv cannulas before and at measured intervals after stimulation. With the exception of a control group of 4-day cyclers bled sequentially on the proestrus afternoon to evaluate the normal LH surge, CD rats were bled only once, by cardiac puncture 90 min after stimulation, the interval when highest serum LH concentration was attained in most O-M rats. The oviducts were searched for ova next morning. Serum LH was assayed by the ovine:ovine technique. Approximate cross-sectional areas of most of the stimulative lesions were measured from camera lucida tracings. There was a highly significant direct relationship between lesion area and the amount of circulating LH at 90 min in both proestrous and D-3 rats, in both strains. LH levels following stimulation were significantly lower on D-3 than on the day of proestrus. O-M rats stimulated during proestrus presented significantly larger amounts of LH at 90 min than did CD rats. The numbers of animals ovulating in response to the smaller stimuli was somewhat lower than expected from past experience, suggesting a deleterious effect of the blood withdrawal. (Endocrinology93: 947, 1973)