The potential role of basic fibroblast growth factor (bFGF) as a mediator of cell-cell interactions in the growth and development of the testis was examined. Nuclease protection analysis was used to evaluate bFGF gene expression in the testis and other male reproductive tract tissues. bFGF expression was evident in seminal vesicle, prostate, epididymis, and, at low levels, testis of 20-day-old rats. The developmental expression of bFGF in whole testis and isolated somatic cells types was determined. Mesenchymal-derived peritubular cells and epithelial-like Sertoli cells were isolated from prepubertal, midpubertal, and late pubertal rat testes. In whole testis, bFGF expression is predominant early in prepubertal testicular development and decreases with sexual maturity. Both freshly isolated peritubular and Sertoli cells express bFGF at relatively constant levels during pubertal development, with a slight suppression at the late pubertal stages. Freshly isolated mature Leydig cells also expressed low levels of bFGF. Cultured Sertoli and peritubular cells produced bFGF-like proteins, including 18- and 24-kilodalton forms. Interestingly, FSH increased Sertoli cell bFGF gene expression and protein production. Previously, FSH and bFGF have been shown to stimulate immature Sertoli cell growth. The results of the current study suggest that the ability of FSH to regulate testis and Sertoli cell proliferation may in part be indirectly mediated through the local production and action of bFGF. bFGF has also previously been shown to localize in developing germinal cells. Therefore, FSH-induced Sertoli cell bFGF expression may mediate Sertoli-germinal cell interactions involved in the control of the spermatogenic process. Observations demonstrate the presence of bFGF at a time coinciding with active growth of the somatic cell populations of the seminiferous tubule. Potential roles for bFGF in the seminiferous tubule to consider include angiogenesis of the tubule, prepubertal Sertoli cell proliferation, and mediating Sertoli-germinal cell interactions.