The physiological importance of the increase in androgen receptors in exercise-induced muscle hypertrophy was investigated in rats. Together with training rat gastrocnemius muscles by electrical stimulation every other day for 2 weeks, male rats were administered the androgen receptor antagonist, oxendolone. The androgen receptor antagonist effectively decreased the wet mass of the prostate, an androgen target organ, and did not significantly affect body mass. The increase in muscle mass induced by electrical stimulation was effectively suppressed by the androgen receptor blockade. The mean degree of muscle hypertrophy in the antagonist-treated group was significantly lower than that in the control group (102.30% vs 107.41%, respectively; P = 0.006). This result suggests that the androgen pathway has a significant effect in exercise-induced muscle hypertrophy and emphasizes the importance of the increase in the number of androgen receptors in exercised muscle.