Adult [female] castrated guinea pigs were injected with 1 mg. of testosterone propionate daily for 26 (2 animals) or 34 (6 animals) days. Endometrial specimens from these animals were compared with similar specimens from adult guinea pigs which had received 2 R.U. of theelin daily for 21 (17 animals) or 30 to 40 (8 animals) days. Endometria from guinea pigs injected with testosterone showed typical dilatation of the glands, moderate proliferation of the surface epithelium, mild proliferation of the glandular epithelium and a mild reaction of the stroma. Endometria from guinea pigs injected with theelin showed typical swiss-cheese dilatation of the glands, marked proliferation of the surface and glandular epithelium and a strong reaction of the stroma. The response of the endometrium to testosterone was qualitatively similar to the response to theelin, but the 2 differed in potency; cystic dilatation of the endometrial glands might result from the application of a stimulus which was, comparatively speaking, extremely weak.