The factor(s) which trigger the proliferation of liver cells after partial hepatectomy in rats are not yet known with certainty. A series of specific and/or non-specific substances are said to effect the proliferation of liver cells, but metabolic changes after partial hepatectomy possibly also act as a contributory causative factor in the stimulation of those substances. In order to define this question more clearly the changes in the serum concentrations of the following metabolites in the course of time after partial hepatectomy were determined: free fatty acids, cholesterol, triglycerides, glucose, free amino acids and total protein. A reduction in the serum concentrations of some fatty acids and of glucose, and an increase in that of the free amino acids, was observed shortly after partial hepatectomy. The increase in the plasma concentration of immunoreactive glucagon after partial hepatectomy is thus explained and appears not to be of decisive significance for the stimulation of liver cell proliferation after partial hepatectomy.