Fetal rat brown adipocytes at the beginning of culture showed a minimal T3 nuclear binding capacity and very low expression of the c-erbA genes, with low steady state levels of the mRNA forms beta-type and 5.5- and 2.6-kilobase (kb) alpha-types. The levels of these mRNA species increased after 7 days in culture in the presence of thyroid hormone-depleted serum; however, no significant increase in nuclear T3-binding capacity was observed. The addition of T3 incremented the abundance of the c-erbA beta-mRNA, induced the appearance of the 6.6-kb c-erbA alpha-mRNA and increased the nuclear T3-binding capacity by 30-fold. Parallel analysis of the uncoupling protein (UCP) mRNA, immunoreactive UCP, and functional UCP (detected by its ability to bind GDP) demonstrated that T3 induced the expression of the UCP gene in long term treated fetal brown adipocytes in culture. The presence of noradrenaline, a positive control for UCP expression, increased only the level of expression of the 5.5-kb c-erbA alpha-mRNA, but values for nuclear T3-binding capacity similar to those obtained in T3-treated cells were observed. Simultaneous addition of both hormones gave a pattern of expression of the c-erbA-mRNA forms similar to those obtained with these agents individually, and no further increment in their separate effects was observed on the nuclear T3-binding capacity and UCP expression.