The biosynthesis of thymidylate in the yeast Saccharomyces cerevisiae can be inhibited by antifolate drugs. We have found that antifolate treatment enhances the formation of leucine prototrophs in a haploid strain of yeast carrying, on the same chromosome, two different mutant leu2 alleles separated by Escherichia coli plasmid sequences. That this effect is a consequence of thymine nucleotide depletion was verified by the finding that provision of exogenous thymidylate eliminates the increased production of Leu+ colonies. DNA hybridization analysis revealed that recombination, including reciprocal exchange, gene conversion and unequal sister-chromatid crossing over, between the duplicated genes gave rise to the induced Leu+ segregants. Although gene conversion unaccompanied by crossing over was responsible for the major fraction of leucine prototrophs, events involving reciprocal exchange exhibited the largest increase in frequency. These data show that recombination is induced between directly repeated DNA sequences under conditions of thymine nucleotide depletion. In addition, the results of this and previous studies are consistent with the possibility that inhibition of thymidylate biosynthesis in yeast may create a metabolic condition that provokes all forms of mitotic recombination.