The present study was designed to determine if a fraction with MW > 30 KDa secreted by adult rat seminiferous tubules in vitro has any effect on fertility when administered in vivo to adult male rates, and on inhibin secretion when added to the culture medium containing adult rat seminiferous tubules in vitro. The addition of the fraction with MW greater than 30 KDa resulted in an inhibition of inhibin secretion into the medium containing the seminiferous tubules cultured by two days. The effect was reversed when the fraction was removed and changed with fresh medium and cultured for further four days. The fraction administered intraperitoneally to adult males rates reduced the rate of fertilization from 67% to 25% (P < 0.05). The study of the frequencies of stages determined by transillumination technique showed a reduction of the stages VII and VIII, with a concomitant increase in frequency of stage VI. Results of the present study suggest the existence of local factors produced by own germ cells which regulate Sertoli cell function and spermatogenesis.