The effects of thymidine (TdR) co-administration on the cytotoxicity and incorporation of 5-fluorouracil (5-FU) into RNA of various tissues was studied in rats bearing an ascites hepatoma (AH 130). The role of pyrimidine degradation in determining the modulating effects of TdR on the formation of FU-RNA was studied in hepatocytes and AH 130 cells in vitro. TdR (500 mg/kg) potentiated the antitumour effect of 5-FU (150 mg/kg) and also increased host toxicity as judged by changes in body weight. TdR given alone did not significantly affect tumour growth and body weight gain. Examination of the effect of TdR on the incorporation of 5-FU into RNA revealed a differential modulation of RNA-directed toxicity in different tissues. Incorporation of 5-FU into RNA in tumour and bone marrow was increased 2- and 4-fold, respectively. In spleen and kidney the incorporation increased by approximately 50%, but the values did not reach statistical significance. In contrast, the incorporation into RNA of liver and intestinal mucosa was decreased to ca 35% of the control. TdR at concentrations of 40 microM-40 mM progressively inhibited the degradation of 5-FU and decreased the incorporation of 5-FU into RNA of hepatocytes in vitro. In AH 130 cells in vitro TdR did not significantly influence the metabolism of 5-FU and the incorporation into RNA. These results demonstrate that the enhanced incorporation of 5-FU into tumour RNA in vivo after pretreatment with TdR is related not to local effects on the tumour cells but rather to an increased bioavailability of the drug. Although co-administration of TdR did not selectively enhance the antitumour effect of 5-FU, a differential toxicity in host tissues was indicated by the modulated incorporation of 5-FU into RNA.