The known stimulating effect of calf serum (10%) and cortisol (20 microgram/ml) on the incorporation of 2H uridine into the nuclei of isolated rat hepatocytes has been confirmed with the method of light and electron autoradiography. Moreover, it has been shown that cortisol and serum exert additive effects when they are applied jointly. In the presence of either serum or cortisol or both transcriptional activity increases gradually, reaching its maximum level after 6 h incubation. This effect is reduced when the incubation is prolonged up to 24 h. Autoradiographic ultrastructural studies have shown that both serum and cortisol enhance the nucleolar as well as extranucleolar transcriptional activity. The nuclei of hepatocytes cultivated in medium containing serum or both serum and cortisol shown a higher growth rate in comparison with those cultivated in a poor, control medium. Cortisol added to the serum-free medium does not stimulate the nuclear growth.