Hydroxyurea (HU) causes inhibition of DNA synthesis in regenerating rat liver due to an inhibition of the ribonucleotide reductase. We studied the consequences of a continuous HU infusion for deoxyribonucleoside triphosphate (dNTP) pools in the liver after partial hepatectomy and tried to modify imbalances by application of deoxyribonucleosides in vivo. In normal liver, an intracellular concentration of 0.16, 0.84, 0.33 and 0.27 pmol/micrograms DNA was observed for dATP, dCTP, dGTP and dTTP, respectively. In regenerating liver the dNTP pools show minor changes until 18 h after partial hepatectomy. During and after a continuous HU infusion 14--24 h after partial hepatectomy, the intracellular dNTP pools change considerably. At 19.5 h after partial hepatectomy, 5.5 h after the start of HU infusion, and at 25 h after partial hepatectomy, 1 h after termination of HU infusion, the dTTP pool was more than 10-times, and the dGTP pool about 2-times higher than in controls, while the dATP and dCTP pools remain relatively unchanged. Simultaneous infusion of HU and deoxythymidine (dThd) 14--25 h after partial hepatectomy results in a further increase of the dTTP pool during and after HU infusion. Administration of deoxycytidine (dCyd) leads to a moderate increase of the dCTP pool and a weak decrease of the dTTP pool during HU infusion. The combined application of dCyd and dThd after HU infusion had similar effects on dNTP pools as observed with dThd alone. These results show that intracellular pools of dNTPs in hepatocytes can be altered by exogenous factors in a controlled pattern. This system can be used as a model for studying the implications of induced dNTP pool dysbalances for the initiation of liver carcinogenesis by mutagenic chemicals.