The influences of uterine traumatization, ovariectomy and hormone replacement therapy on decidual alkaline phosphatase were studied in pseudopregnant rats. The enzyme was also assayed in implantation sites of pregnant rats with unilateral section and ligation of the Fallopian tubes. In all pseudopregnant animals with traumatized uteri, intact or castrate receiving progesterone replacement, the enzyme activity was 12 times more active on day 8 of pseudopregnancy than the contralateral non-traumatized horn. The alkaline phosphatase reaction of day 8 pregnant rats demonstrated a similar response at the implantation site. No increased enzyme was elicited by pseudopregnant rats treated with 17β-oestradiol and/or progesterone or animals ovariectomized and traumatized on day 4 and given only oestrogen. Additionally, the changes in alkaline phosphatase from day 5 to 10 in the intact and hormone-treated, ovariectomized pseudopregnant rats with traumatized uteri were similar to those demonstrated by the intact pregnant animals, i. e., an increased reaction from day 5 to 8 and decreased activity from day 8 to 10. The data support the view that the rise in decidual alkaline phosphatase in pregnant or traumatized pseudopregnant animals is dependent on the combined effect of uterine irritation and a progesterone-conditioned endometrium.