A model for the in vitro study of the conversion of thyroxine into triiodothyronine using isolated rat liver parenchymal cells is described. Isolated liver cells (mean protein content 18 mg/ml convert approximately 0.8% of 1.3 μM exogenously added T4 into T3 during thirty minutes incubation. Carbimazole (50 μM) has no effect on the conversion process, whereas propylthiouracil (50 μM) inhibits the conversion. The β-adrenoceptor blocking agent propranolol lowers the conversion ratio when added in concentrations of 580 and 1160 μM, but has no inhibitory effect when 290 μM is added.