Background: Anabolic androgenic steroids (AAS) use by professional and recreational athletes is increasing worldwide. Aim of the work: This study inspected the postmortem changes within the first 24 h in heart of mesterolone-treated rats. Material and Methods: 120 adult male albino rats were divided into two groups; Group I (control): 80 rats, subdivided into two equal subgroups; Group II (Mesterolone) (40 rats): treated with 2.14 mg/kg B.W mesterolone suspended in 1ml corn oil. All treatments were orally gavaged once daily for twelve weeks. Rats were euthanized, and the heart specimens (ventricles) were collected at zero-time, 4, 12, and 24 h PM for biochemical and histopathological investigations. Results: Mesterolone significantly increased pH, MDA, ATP, ADP, and glycogen values as well as reduced the GSH and lactic acid levels when compared to the control groups. A postmortem time-dependent significant drop in cardiac muscles pH, antioxidant marker, ATP, ADP, and glycogen levels, contrariwise a significant rise in lipid peroxidation and lactic acid concentration were recorded in both control and mesterolone groups. A progressive PM histological deterioration and collagen degradation were detected while; mesterolone significantly increased the collagen fibers reaction in cardiac muscles. Conclusion: These findings point out mesterolone exposure's modifying effect on the energy content, oxidative status, and histological architecture of cardiac muscles in the early PMI. Outcomes should be carefully considered when determining PMI in cases with a history of AAS abuse or patients pretreated with these medications for long durations or sudden death in young healthy adults.