The enzymes which comprise the 2′,5′-oligoadenylate synthetase (OAS) family are interferon (IFN) stimulated genes which regulate ribonuclease L antiviral responses and may play additional roles in control of cellular growth and differentiation. This study characterized OAS expression in the endometrium of cyclic and pregnant ewes as well as determined effects of IFNτ and progesterone on OAS expression in cyclic or ovariectomized ewes and in endometrial epithelial and stromal cell lines. In cyclic ewes, low levels of OAS protein were detected in the endometrial stroma (S) and glandular epithelium (GE). In early pregnant ewes, OAS expression increased in the S and GE on Day 15. OAS expression in the lumenal epithelium (LE) was not detected in uteri from either cyclic or pregnant ewes. Intrauterine administration of IFNτ stimulated OAS expression in the S and GE, and this effect of IFNτ was dependent on progesterone. Ovine endometrial LE, GE, and S cell lines responded to IFNτ with induction of OAS proteins. In all three cell lines, the 40/46-kDa OAS forms were induced by IFNτ, whereas the 100-kDa OAS form appeared to be constitutively expressed and not affected by IFNτ. The 69/71-kDa OAS forms were induced by IFNτ in the S and GE cell lines, but not in the LE. Collectively, these results indicate that OAS expression in the endometrial S and GE of the early pregnant ovine uterus is directly regulated by IFNτ from conceptus and requires the presence of progesterone.