Nuclear T3 binding and T3 receptors (TR) expression were studied in brown adipocytes differentiated in culture from precursor cells. High affinity T3 receptors were found. During adipocyte differentiation maximal binding capacity (MBC) was doubled (up to 763 fmol T3/mg DNA), and an apparent decrease in receptor affinity was also observed (due to a decrease in the association rate constant). A very high disappearance rate of T3 was found in the cellular and nuclear fractions under binding assay conditions (serum-free medium). MBC increased 30% under hypothyroid conditions, but was not affected by physiological doses of T3 or retinoic acid. TRβ1, TRα1 and c-erbA-α2 mRNAs were detected in brown adipocytes. During differentiation TRβ1 decreased to 30%. Long exposure to T3 increased 2-fold TRβ1 and decreased TRα1 levels, when using insulin-depleted medium. Short exposure (4 h) to 10 nM insulin reduced both TRβ1 and TRα mRNAs species.