Adrenalectomized female mice (CR albino and a wild strain) were exposed to strange males 24 hr after insemination. Exposure of adrenalectomized CR females had no demonstrable effect while sham-operated CR females had a significant number of pregnancies blocked by this treatment. Wild females were considerably more sensitive to the strange males. Adrenalectomy again acted to protect against pregnancy block but not to the same degree as in the laboratory strain. It was concluded that the pituitary-adrenal axis is involved as part of the endocrine mechanisms blocking pregnancy.