We have tried to determine the number of genes for the rat liver glutathione S-transferase Yc, using specific probes prepared by polymerase chain reaction. DNA blot hybridization with specific probes and analysis in isolated genomic clones indicated that two copies of the Yc gene exist on the rat genome. Furthermore, DNA blot hybridization after clamped homogenous electric field gel electrophoresis indicated that the two copies of Yc gene family are closely located on the genome.