Deoxycytidine improves tolerance of Drosophila melanogaster to thymidine block, suggesting the presence of deoxycytidine kinase. At appropriate concentrations, a mixture of thymidine and deoxycytidine allows larvae to tolerate a higher concentration of 5-fluoro-2′-deoxyuridine than is tolerated with either thymidine or deoxycytidine alone. Thus, at this high concentration, 5-fluoro-2′-deoxyuridine appears to act primarily upon thymidylate synthetase, as it does at lower concentrations, rather than upon RNA metabolism, as has been suggested previously. Larvae can also be rescued from 5-fluoro-2′-deoxyuridine-induced death by a high concentration of thymine. The effect is enhanced by the presence of deoxyadenosine. Since this compound is known to increase the intracellular concentration of deoxyribose-1-phosphate, the main effect of thymine is probably due to its salvage utilization as a thymidine source, via the anabolic functioning of thymidine phosophorylase. © 1980.