Since the development of a maternal pregnancy reaction at times during gestation other than day 12 was not investigated, the present study was undertaken to see whether a sensitive period existed in ferrets. Histological examination of traumatized endometrium in 1 experiment showed that a maternal pregnancy reaction developed only at certain stages of gestation. Histological changes appeared most pronounced when the intrauterine thread was inserted on day 12. No specific response was elicited before day 6 or after day 14; traumatization of the endometrium outside the sensitive period produced occasional patches of surface necrosis involving not only the epithelium and connective tissue but also the capillaries. The period of sensitivity occurred only between days 9-14 in a 2nd experiment. The results of the 1st experiment do not exclude the possibility that the endometrial response to a thread inserted before day 12 is due to the presence of that thread on or around day 12, but the results of the 2nd experiment show that uterine sensitivity is restricted to a relatively short period around day 12. Production of a maternal pregnancy reaction in the ferret is probably progesterone-dependent.