The direct effect of Tri-iodothyronine (T(3): 0.1-100 nM) on protein turnover was studied using primary cultures of Sertoli cells isolated from immature piglet testis. The results demonstrate that T(3) significantly increases protein synthesis without altering the protein degradation rate. These data and previous ones, showing the presence of specific T(3) receptors in Sertoli cell nuclei, indicate that T(3) plays a fundamental role in the early regulation of porcine Sertoli cell growth and maturation.