Anabolic-androgenic steroids (AASs) such as nandrolone decanoate are used by some athletes to enhance performance and improve athletic ability, physical appearance and muscle mass. Despite the health risk, they may pose in some persons. This work was carried out to evaluate the possible histological and ultrastructural alterations in the cardiac muscles of adult mice after injecting nandrolone decanoate, as an example of AASs. Twenty adult male CD-I mice were used in the present study; they were divided into two groups, each of 10 animals. Animals of the first group served as control and were intraperitoneally injected with saline solution. Animals of the second group were intraperitoneally injected every second day with 3 mg/kg body weight with nandrolone decanoate (ND) for four weeks. At the end of the experimentation, all the animals were sacrificed and cardiac muscle samples were obtained and processed for histological and ultrastructural examinations. Light microscopy examination of the cardiac muscle fibres of the treated mice showed focal areas of degeneration with loss of striations, vacuolated cardiomyocytes, congested blood vessels and scattered mononuclear inflammatory cells amidst the necrotic and degenerated muscle fibres. Ultrastructural examination of the cardiac muscle fibres of the treated animals showed disturbance and disarray of the banding pattern of the cardiac muscle fibres with loss of striations, cytoplasmic vacuolation, and aggregation of pleomorphic mitochondria and enlarged nuclei with irregular envelopes. Increased number of fibroblasts was also noticed. So, the administration of AASs caused severe deleterious effects in the myocardium of the experimental animals.