Diencephalic structures that influence plasma thyrotropin (TSH) in male rats under pentobarbital anesthesia (35 mg/kg, IP) were studied by combining medial preoptic area-suprachiasmatic nucleus (MPOA-Sch) bilateral electrical stimulation (monophasic pulses, 200 microA at 50 Hz, 30 min) with progressive midline lesions produced by a retractable Halász knife. Plasma TSH was measured by radioimmunoassay just before (0 time) and at 30, 60 and 90 min after the beginning of stimulation. Rats that had received only sham surgical procedures 90 days prior to stimulation were characterized by a more than 2-3 fold elevation in basal (0 time) plasma TSH levels when compared to those found in intact control rats and expected elevations in plasma TSH at 30 min after stimulation were eliminated. After a small frontal cut (1.3 FC), 0 time plasma TSH levels increased more tha 4-fold above those of controls in association with a facilitation of stimulated release of TSH. When the knife blade radius was 1.5 mm (1.5 FC) the facilitation of TSH release after stimulation occurred again; however, 0 time plasma TSH concentrations in 1.5 FC rats were not different from control levels. These effects of midline cortical, thalamic and hypothalamic damage on TSH release required the passage of more than 12 days after brain surgery. Collectively, these findings suggest potential neural elements that are inhibitory for 'basal' and 'phasic' TSH release which are in close proximity to a separate excitatory neural system and which can be activated by MPOA-Sch stimulation.