To study the effect of human growth hormone (hGH) on liver regeneration in rats, 200 micrograms hGH was administered to partial hepatectomized rats twice a day for three days. The bw of hGH-treated rats was higher than that in untreated rats. After three day administration, the liver weight was 3.18 +/- 0.13 g, significant higher than that of untreated rats (2.68 +/- 0.17 g). Human GH also stimulated the mitosis in the liver. Serum insulin-like growth factor I (IGF-I) and albumin levels were significantly increased and urea nitrogen levels were significantly decreased in hGH-treated rats compared with those in untreated rats. When 120 micrograms/day IGF-I was continuously administered to partial hepatectomized rats for three days, the bw and the liver weight were not higher than those of controls. These data indicate that hGH directly stimulates liver regeneration and recover liver dysfunction in rats.