Regulation of 9.5-kb androgen receptor mRNA concentrations in Sertoli and peritubular cells from 20-day-old rats was studied by Northern blot analysis. Treatment of cells in vitro for 1-7 days with 300 ng/ml FSH increased androgen receptor mRNA up to 4-fold in Sertoli cells but not in peritubular cells. Testosterone (100 ng/ml) had no effect or slightly decreased androgen receptor mRNA in Sertoli and peritubular cells. Androgen receptor mRNA concentrations in Sertoli and peritubular cells from rats killed 15 days after hypophysectomy were elevated 4-5-fold over those in cells from intact rats. The androgen receptor mRNA concentration was decreased in both Sertoli and peritubular cells isolated from hypophysectomized animals treated with 500 micrograms/day testosterone propionate in vivo and subsequently with 100 ng/ml testosterone in vitro. FSH treatment (100 micrograms/day in vivo, followed by 300 ng/ml in vitro) did not increase androgen receptor mRNA over that in cells from hypophysectomized controls but rather decreased its concentration to varying degrees in Sertoli and peritubular cells. The rise in androgen receptor mRNA in both Sertoli and peritubular cells isolated from hypophysectomized animals is attributable, at least in part, to the absence of the inhibitory influence of testosterone. Other data in the literature suggest positive regulation of Sertoli cell androgen receptor protein by FSH and androgens. Consequently, complex mechanisms involving transcriptional, translational, and post-translational regulation probably control androgen receptor concentrations in the cells of the rat seminiferous tubule.