The H2-blockers, cimetidine and ranitidine, are reported to inhibit liver regeneration. In this work, the effects of a new powerful H2-receptor antagonist, famotidine, on liver regeneration were studied in rats. The animals were divided into three groups: group I in which a standard two-thirds hepatectomy was performed, group II in which the rats were treated with famotidine (intramuscular dose of 0.8 mg/kg body weight) on the day of operation and 24 and 48 h after hepatectomy, and group III in which the animals were intramuscularly injected with a larger dose of famotidine (1.2 mg/kg body weight) in the same way as group II. The histology and mitotic index of remnant livers and serum levels of aminotransferase and albumin were examined from 24 h to 10 days after the operation. The treatment with famotidine (groups II and III) did not inhibit hepatocyte mitosis but, on the contrary, raised the index on day 3 after hepatectomy when compared with the controls (group I). The albumin synthesis was well preserved in the famotidine-treated animals. The noninhibitory effects of famotidine on liver regeneration are discussed.