The mRNA for basic fibroblast growth factor has been detected in primary cultures of sheep thyroid cells. RNA species of 2.0 kb, 3.3 kb and 7.2 kb have been identified. The effects of this autocrine factor on the production of insulin-like growth factor binding proteins (IGFBPs) have been examined. bFGF enhanced the secretion of IGFBP-2 and, like epidermal growth factor (EGF) and the tumour promoting phorbol ester (TPA), induced the appearance of IGFBP-3. We conclude that bFGF is synthesized by sheep thyroid cells and has autocrine potential since it can regulate the secretion of other thyroid autocrine factors which, in turn, may regulate thyroid growth and function.