Gyrate atrophy is a hereditary chorioretinal degenerative disease caused by a deficiency of the mitochondrial enzyme, ornithine aminotransferase (OAT). Recent investigations have demonstrated the molecular genetic defects of OAT in gyrate atrophy patients. We constructed a eukaryotic expression vector (pcDHOAT) which contains the SV40 promoter and human OAT cDNA. We used OAT(-) Chinese hamster ovary (CHO) cells, which have negligible OAT activity, and fibroblasts from a gyrate atrophy patient (GA35 cell), which have negligible OAT mRNA and enzyme. Incorporation of pcDHOAT and synthesis of human OAT mRNAs and active enzyme were demonstrated in both cell types. The level of expression of human OAT was low in the GA35 cells in comparison to the CHO cells. Despite the limited success, the ability to express active OAT in these OAT-deficient cells using an expression vector offers possibilities of replacement gene therapy for gyrate atrophy.