Primary cultures of adult rat hepatocytes are stimulated to enter DNA synthesis by norepinephrine (NE). This stimulation is maximal if the hepatocytes are incubated with NE for more than 12 hr, beginning no later than 2-4 hr after the cells are first plated. After 24 hr in culture, hepatocytes are unresponsive to NE stimulation. A strong synerergistic interaction between NE and epidermal growth factor (EGF) may be observed in cultures incubated with both EGF and NE, or pretreated with NE, then exposed to EGF. This interaction may be related to the finding that NE, in similarity with other factors that enhance EGF stimulation, reduces binding at the EGF receptor during the first 24 hr in culture. (Copyright (c) Alan R. Liss, Inc.)