The effect of long term intraportal infusion of insulin on liver regeneration has been studied in rats 24, 36 and 48 hrs after partial hepatectomy. Our studies showed an enhancement by insulin of some parameters of rat liver regeneration, i.e. a rapid increase of the DNA and RNA levels as well as the TTK activity in the whole liver homogenate and all the subcellular fractions examined. The influence of insulin was marked during the whole time of experiment, but the most significant changes occur during the first 24 hrs after partial hepatectomy.