Thyroid hormone (T-3), a known stimulator of renal proximal tubular brush border membrane Na-dependent phosphate (P-i) uptake (Na/P-i-cotransport), stimulated Na-dependent P-i transport in opossum kidney (OK) cells. Na/P-i-cotransport was stimulated in a time- and dose-dependent manner with maximal effects (57%) at 24 h and 10(-10) M T-3. This stimulation was related to an increase in the apparent capacity (V-max) of Na/P-i-cotransport. Treatment with T-3 had no effect on Na-independent transport of P-i or of L-arginine. The stimulation of Na/P-i-cotransport was paralleled by an increase in the messenger ribonucleic acid (mRNA) encoding the OK cell apical Na/P-i-cotransporter (termed NaPi-4); the mRNA levels related to the activity of Na-independent L-arginine transport (rBAT) were unaffected by T-3. Actinomycin D (10(-7)M) completely prevented the stimulatory effect of T-3 On OK cell Na/P-i-cotransport and on NaPi-4 mRNA content. In conclusion, T-3 stimulates apical Na/P-i-cotransport in OK cells most likely by enhancing its transcription