It was known that the regenerating liver cytosol accelerated liver regeneration, whereas normal liver cytosol inhibited it during first 28 hours, but promoted it later by the thymidine incorporation test. In experiments, Sprague-Dawley male rats were divided into 3 experimental groups (A, B and C) which had 10 rats in each. Also, each group was equally subdivided into 2 subgroups such as A-8 and A-28 in the A group, B-8 and B-28 in the B group and C-8 and C-28 in the C group. All groups were partially hepatectomized equally. The remnant livers were procured at 8 hours in the 3 subgroups A-8, B-8 and C-8 and at 28 hours in the other 3 subgroups A-28, B-28 and C-28. Another experiment was performed in 10 rats under the same partial hepatectomy to get the average weight of the remnant liver. These data could be taken as the initial weight of the remnant liver in the above experimental animals. The normal saline, normal liver cytosol extract and regenerating liver cytosol extract were injected intraperitoneally after the partial hepatectomies, 4 ml of in the A, B, and C groups, respectively. Since specific structural changes might precede demonstrable functional alterations, morphometric measurements were done in the histological specimens of the 3 subgroups A-8, B-8, and C-8. The remnant livers, obtained in all six subgroups, were weighed to calculate the regeneration rates. The overall results are as follows : 1) The liver regeneration rates in the six subgroups were 4.1% in A-8, 3.8% in B-8, 4.3% in C-8, 28.4% in A-28, 16.7% in B-28, and 32.8% in C-28. 2) The morphometric study showed that the nucleus/cytoplasm ratios in the three subgroups were 0.1389 in A-8, 0.2048 in B-8, and 0.3705 in C-8. These results mean that regenerating liver cytosol extract promotes liver regeneration and that normal liver cytosol extract inhibits it during first 28 hours after a partial hepatectomy, but promotes cell division for subsequent regeneration.