The contribution of basic fibroblast growth factor to brown adipose tissue (BAT) enlargement during cold acclimation was investigated using rat brown adipocytes in primary culture. After cold exposure (at 5 degrees C) for 28 days, the level of bFGF messenger ribonucleic acid (mRNA) in BAT of cold-acclimated rats was markedly increased with the increase in the BAT weight. In addition, the blood plasma from cold-acclimated rats considerably enhanced the expression of basic fibroblast growth factor mRNA in rat brown adipocytes. Likewise, the blood plasma from cold-acclimated rats significantly stimulated the growth of rat brown adipocyte precursor cells compared with that from warm-acclimated rats, whereas there was no difference of effect between the two blood plasmas on the growth of bovine capillary endothelial cells. Basic fibroblast growth factor, but not platelet-derived growth factor stimulated the growth of brown adipocyte precursor cells. The conditioned medium from brown adipocyte primary culture markedly stimulated the growth of bovine capillary endothelial cells and the effect was inhibited considerably by antibasic fibroblast growth factor antibody. These results suggest that some factors concerned with the growth of brown adipocyte precursor cells are present in the blood plasma from cold-acclimated rats, and that basic fibroblast growth factor produced by brown adipocytes may significantly contribute to BAT enlargement by autocrine mechanisms during cold exposure.