A hereditary disease in mink (Mustela vison Schreb.) leading to death when the affected kits are about six weeks old has been investigated. The disorder is inherited as a simple autosomal recessive character. Strongly elevated plasma tyrosine concentration is an outstanding feature of the disease. An enzyme defect in tyrosine aminotransferase (EC 2.6.1.5) or 4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27) is considered together with the possibility of a parallel between the disease in mink and the disease tyrosinosis or hereditary tyrosinemia in man.