The initiation of DNA synthesis has been studied in quiescent primary cultures of fetal rat hepatocytes using defined hormones and chemically defined medium. Preparations of crystalline insulin (0.01-10 microg/ml) or 20,000-fold purified somatomedin (0.01-1 microg/ml) are stimulatory. Growth hormone (0.025 microg/ml) and hydroxycortisone (0.025 microg/ml), 3':5'-GMP! (10(-5) M) fail by themselves to initiate DNA synthesis but added together with insulin, enhance the stimulatory response by 50-100%. Thyroid hormones (L-T(3), L-T(4), 7.5-30 ng/ml) are by themselves without effect, but when they are added to thyroid hormone-depleted serum, the reconstituted mixtures show an enhanced capacity to initiate DNA synthesis. In contrast, glucagon (0.01 microg/ml) inhibits the insulin-stimulated response by about 50% without reducing basal DNA synthesis rates. The results described here and in the accompanying two reports indicate that environmental control over the initiation of DNA synthesis is complex, and can involve the participation of many factors. The in vitro findings are consistent with the concept that liver regeneration is hormonally controlled and raise the possibility that alterations of the intrahepatic ratio of insulin to glucagon are growth regulatory.