Abstract: Testosterone propionate (TP)was demonstrated to have an action of terminating early pregnancy in mice. The ED₅₀ of TP to terminate early pregnancy was 0.11±0.03 mg/kg. This effect of TP was shown to be reduced by the concomitant administration of megestrol acetate (MA) or HCG. TP at doses large enough to terminate early pregnancy was found to significantly inhibit the decidual cell reaction in pseudo-pregnant mice. This inhibitory effect was abolished, by MA. In contrast, TP could not inhibit the decidual reaction in ovariectomized mice treated with estrogen and progestin. TP showed no luteolytic effect, nor did it potentiate the effects of oxytocin and 15-methyl-PGF_2α on the uterine smooth muscle.It would appear that the effect of TP to terminate early pregnancy is associated with its inhibitory effect on the decidual reaction, and that the inhibitory effect of TP on the decidual cell reaction was attributable to its inhibiting effect on the hypothalamus-pituitary-ovary axis but not due to its direct effect on the deciduoma.