AbstractPreviously it had been demonstrated that daily injections of testosterone propionate (T.P.) will not induce male‐like mounting nor clitoral hypertrophy in pregnant guinea pigs to the extent it will in non‐pregnant females. In addition, administered progesterone had been shown effective producing a similar behavioral, but not somatic, “protection.” In an attempt to clarify the relationships of endogenous progesterone producing organs and this antiandrogen protective phenomenon the behavioral and somatic responses of females ovariectomized while pregnant and females hysterectomized (made pseudopregnant) while non‐pregnant were studied.Guinea pigs were hysterectomized on the sixth day of their ovarian cycle and ovariectomized 60 days later. Additional groups of females were mated and then ovariectomized on the thirty‐sixth or forty‐sixth day of gestation. Groups of normal pregnant females, aborted females, and sham operated controls were also studied.All groups received daily injections of T.P. starting on the eighteenth day of pregnancy and continuing for the duration of the experiment (116 days). With these androgen treatments masculinization or protection from maculinization was evaluated on the basis of periodic sex behavior tests and on the basis of daily inspection of the external genitalia.The results indicate that protection from testosterone propionate‐induced virilization is offered by exogenously administered progesterone as well as by an endogenous progestagen‐producing structure such as the corpora lutea of the ovary in a hysterectomized female or the placenta in a pregnant animal. Some factor other than progesterone production is involved and the neural and somatic tissues exhibit different re