The effects of three of the most widely used histamine H2-receptor antagonists, cimetidine, ranitidine and famotidine, on liver cell growth were studied in vitro using adult rat hepatocytes in primary culture, because these antagonists are commonly given to patients with hepatic cirrhosis or fulminant hepatic failure for protection against peptic ulcers and gastrointestinal hemorrhage. At their clinically effective concentrations in the blood (0.5-5 micrograms/ml cimetidine, 0.25-2.5 micrograms/ml ranitidine and 0.05-0.5 microgram/ml famotidine), these three antagonists did not have any effect on replicative DNA synthesis either in the presence or absence of insulin plus epidermal growth factor (EGF). However, unexpectedly DNA synthesis stimulated by insulin and EGF was found to be enhanced by 0.05-0.5 mg/ml cimetidine, although it was unaffected or inhibited by ranitidine and famotidine at the concentrations tested. Cimetidine caused maximal enhancement of 1.5-2 times the control level of DNA synthesis at a concentration of 0.25 mg/ml. Cimetidine also had an enhancing effect at submaximal concentrations of insulin and EGF, but neither cimetidine nor the other antagonists had any stimulatory effect on DNA synthesis in the absence of insulin plus EGF. This enhancement of DNA synthesis by cimetidine resulted in significant increase in the total DNA content of the hepatocytes in culture. Under the conditions used, cimetidine had the lowest toxicity of these three antagonists and ranitidine the highest, as judged from data on DNA synthesis and the total protein content of cultured hepatocytes, leakage of aminotransferases from the cells and morphological observations.