The intensity of gamma-ray-induced unscheduled DNA synthesis (UDS) was measured in a number of natural populations of Vicia cracca which have been growing for some decades under conditions of chronic alpha- or beta-irradiation. It was shown that the level of UDS increased in more radioresistant beta-irradiated populations as compared to control populations and this increase was dose rate-dependent. In alpha-irradiated populations, the intensity of UDS was decreased, but only at the highest dose of gamma-radiation (500 Gy) and was not changed at the lower doses. The sensitivity of UDS to inhibitors of DNA and protein synthesis was studied in control and radioresistant populations. In control plants UDS was resistant to cycloheximide (Cyc) and aphidicolin (Aph), but totally inhibited by dideoxythymidine (ddT). In radioresistant population UDS was inhibited by both Aph and ddT as well as by Cyc. I assume that in controls, UDS is mediated by beta-like DNA polymerase; however, in a radioresistant population, both DNA polymerases alpha and beta take part in this process. In the radioresistant population UDS is partially inducible.