Studies on the metabolism of selected amino acids were carried out in 5 patients with gyrate atrophy of the choroid and retina and 4 obligate heterozygotes. Hyperornithinemia, hyperornithinuria, and hypolysinemia were found in all patients. In one of the patients, the condition was diagnosed as early as 4 yr of age. Ornithine loadings in the affected individuals did not induce the expected elevation of plasma glutamic acid and proline. Oral lysine tolerance tests in patients resulted in enhancement of the hyperornithinuria and hyperlysinuria and elevation of plasma lysine levels, which were below values obtained from normal controls. Supplementation of the regular diet with lysine for a period of 1 mo. increased plasma lysine but had no effect on plasma ornithine concentration.