1. Incorporation in vitro of l-[4,5-3H]leucine and l-[U-14C]lysine into a soluble protein fraction and into actomyosin of hearts, diaphragms and biceps femoris of an inbred strain of Syrian hamsters suffering from a hereditary myocardiopathy and muscular dystrophy was studied.
2. Incorporation of both amino acids was normal in myopathic hearts from hamsters aged 60–90 days but significantly elevated at 180–240 days of age. Their incorporation was higher in the myopathic diaphragm and biceps femoris of both age groups.