Plasma LH and FSH were studied in adult female rats following bilateral electrochemical stimulation (ECS) of the medial preoptic area (MPOA). By stereotaxic surgery frontal (FC) and frontal-lateral (LFC) retrochiasmatic "deafferenting" cuts were made with a Halász knife (1.5 mm radius). At 3 and 10 weeks after surgery rats were given pentobarbital (32 mg/kg, i.p.) at 13:30h and stimulated at 15:00h with anodal direct current (20 muA for 60 sec) via concentric bipolar steel electrodes placed bilaterally 0.9 mm from the midline. Stimulation at 3 weeks after FC increased plasma LH from a prestimulation level of 95 +/- ng/ml to 227 +/- 51 ng/ml 80 min after ECS, from which it fell to 111 +/- 29 ng/ml at 160 min, with 2 of 7 animals ovulating the next day. LFC females had similar pre-ECS plasma LH levels 3 weeks after surgery (71 +/- 10 ng/ml), but LH concentrations at 80 min (104 +/- 21 ng/ml) and 160 min post-ECS were significantly lower than those of FC rats, and 0 of 5 rats ovulated. Following a similar protocol 10 weeks after surgery, stimulating the MPOA resulted in comparable elevations in plasma LH and 4 of 10 FC animals ovulated; however, the LFC group still retained a significant blocking effect on ovulation (only 1 of 12 ovulated) when compared with controls the next day. Plasma FSH concentrations were not significantly altered by MPOA stimulation at the parameters employed, either before or after deafferentation. However, LFC resulted in reduced ovarian and uterine weights when compared with controls at both 3 and 10 weeks, whereas FC exerted no observable effect on these organs at these intervals of study. The results of these studies suggest that lateral input to the media basal hypothalamus contributes to MPOA mediated release of LH and ovulation as well as to tonic maintenance of ovarian and uterine function.