Microsomal epoxide hydrolase plays a dual role in the activation and detoxification of carcinogenic compounds. Two polymorphic sites have been described in exons 3 and 4 of the microsomal epoxide hydrolase gene that change tyrosine residue 113 to histidine (Tyr113His) and histidine 139 to arginine (His139Arg), respectively. The exon 3 polymorphism reduces enzyme activity by approximately 50%, whereas the exon 4 polymorphism causes a 25% increase in activity. In the present study, the distribution of these polymorphisms in a Turkish population including 625 unrelated healthy individuals was examined using a PCR-RFLP method. The observed genotype frequencies of microsomal epoxide hydrolase exon 3 were 54, 38 and 8% for Tyr113Tyr, Tyr113His and His113His, respectively. Exon 4 genotype frequencies were found to be 69, 29 and 2% for His139His, His139Arg and Arg139Arg, respectively.