Chemical mutagens generally cause nucleotide pool imbalance. We postulated that this effect might enhance the mutagenic effect by reducing the accuracy of DNA-repair synthesis. We used an inducer of DNA repair which causes minor pool modifications, namely UV light, and imbalanced the nucleotide pools by incubating UV-irradiated V79 cells with thymidine or deoxycytidine (10(-5)-10(-2) M) during the early phases of repair. The effects on pool sizes of the incubation with deoxynucleosides were determined by directly measuring the 4 deoxynucleoside triphosphates in cell extracts. The impairment of repair accuracy was evaluated by comparing the frequency of mutations at the HGPRT locus (induction of resistance to 6TG) in irradiated cells incubated with deoxynucleosides or allowed to carry on repair synthesis in nucleoside-free medium. Despite the marked imbalance of pyrimidine nucleotide pools, an increase of mutations was observed only with the highest concentrations of thymidine and deoxycytidine. Such an increase was much lower than that reported in the case of facilitation by excess nucleosides of chemically induced mutagenesis. The results indicate that UV-induced repair is scarcely affected by precursor biases.