Objective To investigate the influence of low androgen on myocardial structure of male rats. Methods Male SD rats were randomly divided into control group (n=8), castration group (n=7) and androgen replacement group (n=8). Rats in control group underwent sham castration, those in castration group were castrated, and those in replacement group were given testosterone undecanoate after castration. Ten weeks later, serum concentration of testosterone was measured by radioimmunoassay. Pathological changes of myocardial tissues were observed by light microscopy and electron microscopy. Results Serum concentration of testosterone in castration group was significantly lower than those in control group and androgen replacement group (P<0.01). Light microscopy revealed that in castration group, there were myocardial swelling, hypertrophy, degeneration and nucleus enlargement, the nuclei significantly enlarged, the myocardial fibers fractured, dissolved, contracted and became wavy, and arranged in disorder. Electron microscopy revealed that in castration group, myocardial fibers arranged irregularly, fractured and dissolved, the glycogen and flat drop between muscle fibers accumulated, mitochondria swelled, most of the ridges in mitochondria were not clear, and the intercalated disk widened. Compared with castration group, the above-mentioned pathological changes in androgen replacement group were significantly lessened. Conclusion Obvious myocardial histopathological injury may occur in male rats after castration, and exogenous testosterone replacement therapy may work in some degree. Low androgen may do harm to myocardium of males.