Postnatal Sertoli cell maturation is characterized by a pronounced rise in androgen receptor (AR) expression, which increases several fold between birth and adulthood. Since both 3,3',5-triiodothyronine (T3) and FSH regulate Sertoli cell proliferation and differentiation, we have determined the effects of T3 and FSH on AR mRNA expression in cultured Sertoli cells from 5-day-old rats. These cultures contain 5-9% peritubular cells, which also express AR mRNA. To insure that the observed T3 responses did not result from peritubular cells, we examined T3 effects on AR mRNA expression in cultured 20-day-old Sertoli cells (which contain minimal peritubular contamination) and peritubular cells, and measured thyroid hormone receptor (TR) mRNA expression in both of these cell types. Sertoli cells from 5- and 20-day-old rat testes were grown in serum-free medium alone (controls) or with ovine FSH (100 ng/ml) and/or T3 (100 nM) for 4 days. Peritubular cells purified from 20-day-old rat testes were grown in serum-containing medium for 8 days. These cells were split 1:4, and grown an additional 8 days, the last 4 days in serum-free medium with or without T3. TR and AR mRNA levels in all cultures were determined by Northern blotting. AR mRNA levels in 5- and 20-day-old cultured Sertoli cells were significantly (P < 0.05) increased by both T3 and FSH alone. Furthermore, AR mRNA levels in Sertoli cells treated with T3 and FSH were greater than with either alone. TR mRNA expression was detected in cultured peritubular cells, but TR mRNA levels in these cells were only approximately 30% of that seen in 20-day-old cultured Sertoli cells. In contrast to Sertoli cells, T3 did not affect peritubular AR mRNA expression. These results indicate that T3 is an important regulator of the postnatal Sertoli cell AR mRNA increase. The additive effects of maximally stimulatory doses of FSH and T3 suggest these hormones work through different mechanisms to increase AR mRNA. TR mRNA expression in peritubular cells indicates these cells may be direct T3 targets, though the function of T3 in these cells is unknown.