Type II iodothyronine 5'-deiodinase (5'D-II) activity is the source of 75-80% of the cerebral cortex T3 content in euthyroid rats. The activity of this enzyme is increased in hypothyroidism and can be quickly suppressed by T4 and rT3 by mechanisms involving neither protein synthesis nor nuclear T3 receptors. We have examined the possibility that endogenous cerebrocortical rT3 levels play a physiological role in the regulation of this enzyme. Thyroidectomized rats were injected with graded doses of [125I]rT3, and cortex 5'D-II activity and rT3 content were determined at various times thereafter. Enzyme activity was reduced as early as 10 min after the injection of 0.75 microgram rT3/100 g BW, and 18 h after 25 micrograms/100 g BW remained 60% suppressed. Regardless of the time after the injection, 5'D-II activity was inversely related to the rT3 content in the cortex; nearly complete suppression was observed at 0.5 ng rT3/g tissue, 50% at 80 pg/g, and 20-30% at 30 pg/g, the euthyroid level. After the infusion of 0.75 microgram rT3/100 g, maximal inhibition occurred at 10 min, before the rT3 content reached maximum levels, and the 5'D-II activity started to recover after the rT3 level fell below 300 pg/g tissue. After increasing doses of T4 administered to thyroidectomized rats, serum and cerebrocortical T4 concentrations increased in a dose-dependent manner, but the increment in the latter was steeper than that in the former. Serum rT3 increments were also proportional to the dose of T4, but cerebrocortical rT3 increased to a greater extent, as evidenced by a 3-fold increment in the cerebrocortical rT3 to T4 ratio. With 1.6 microgram T4/100 g BW, cerebrocortical rT3 reached approximately 100 pg/g, about 3 times the euthyroid level, suggesting that at this T4 dose, the rT3 formed from T4 accounts for part of the inhibition of 5'D-II. With the half-maximal suppressive dose of T4, cortex T4 was about 400 pg/g, but rT3 was negligible. We conclude that: suppression of cortex 5'D-II by rT3 is rapid and requires the presence of rT3 in the tissue (i.e. no long-lived mediators); intracortical rT3 is about 5 times more potent than T4 in suppressing this enzyme; the cortex of rT3-5'D-II suppression relationships suggest that the euthyroid levels of cortex rT3 may be significant in the modulation of 5'D-II.(ABSTRACT TRUNCATED AT 400 WORDS)