Glucokinase activity in rat hepatocyte cultures declined with a half-time, t1/2, of 32 h during 3 days under serum-free conditions. Addition of insulin and triamcinolone to the culture medium prevented this decay. Glucokinase levels in hepatocytes derived from fasted rats could be elevated from 7.4 to 16.4 mU/mg protein in the presence of insulin and triamcinolone. In 2-day-old cultures glucokinase was induced in the presence of both hormones with a half time, t1/2, of 5.1 h. In cultures treated for 2 days with triamcinolone, insulin induced a 80% increase of glucokinase even in the absence of glucocorticoids. Insulin induction was dependent on protein synthesis but occurred in the absence of RNA synthesis. Glucocorticoid action, however, depended on RNA synthesis suggesting that glucocorticoids control transcription. Insulin evoked half-maximal effects at 3 nM and dexamethasone and triamcinolone at 0.1 and 1 nM respectively. Degradation of glucokinase was initiated in 2-day-old hepatocytes after removal of triamcinolone and insulin. Protein synthesis was essential for the onset of degradation and glucagon did not affect the rate of glucokinase degradation.