Light microscopical and ultrastructural changes of skeletal muscles were described in chicks treated with monensin and oleandomycin in the food and water for 3 to 6 days. Simultaneously, or slightly subsequent to necrosis of some myofibres on days 3 and 4 of treatment, many myofibres exhibited reversible alterations initiated by focal myofibrillar lysis and degeneration of mitochondria. Reparative changes appearing on day 6 of treatment showed proliferation of the mitochondria, marked increase of ribosomes and polysomes, and enlargement of the Golgi apparatus in the sarcoplasm of degenerated myofibres. The morphological findings of monensin-oleandomycin myopathy in chicks were indistinguishable from monensin-tiamulin myopathy. Possible factors contributing to the unique morphology of this myopathy were discussed.