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. TR beta 1, TR alpha 1 and c-erbA-alpha 2 mRNAs were detected in brown adipocytes. During differentiation TR beta 1 decreased to 30%. Long exposure to T3 increased 2-fold TR beta 1 and decreased TR alpha 1 levels, when using insulin-depleted medium. Short exposure (4 h) to 10 nM insulin reduced both TR beta 1 and TR alpha mRNAs species.