This study was undertaken to evaluate the effect of altered thyroid states on hepatic T3 production in a functioning intact organ system, the isolated perfused liver. Thyroidectomized rats were treated for 3-4 weeks with vehicle, T4, 1.5 micrograms/100 g-1 day-1, or T4, 20 micrograms/100 g-1 day-1, to produce hypothyroidism, euthyroidism, or hyperthyroidism. Livers were perfused for 1 h with medium containing T4, 10 micrograms/dl, and T3 production was estimated by RIA. T3 production in the hypothyroid, euthyroid, and hyperthyroid groups, respectively, was 1.61 +/- (SE) 0.50, 5.18 +/- 0.55, and 15.62 +/- 1.61 ng/g-1 liver h-1. These differences in T3 production resulted entirely from changes in percent conversion of T4 to T3 which were 0.87 +/- 0.25%, 3.21 +/- 0.38%, and 12.02 +/- 1.82% in the hypothyroid, euthyroid, and hyperthyroid groups, respectively. The measured hepatic uptake of T4 decreased slightly with T4 administration from 188 +/- 13 to 162 +/- 7 and 144 +/- 10 ng/g liver in these same groups. The changes in T3 production were not accounted for by differences in biliary excretion or deiodination of T3. These studies demonstrate a stimulatory effect of T4 on the conversion of T4 to T3 which is important to altering net hepatic T3 production.