Normal and chronically inflamed gingival tissue of three beagle dogs were removed by means of gingivectomy for histological and biochemical studies on the in vitro metabolism of female sex steroids, progesterone and estrone. In normal gingiva no conversion of progesterone took place, while in the inflamed tissue 51974- and 51974-pregnanediones were detected as the only conversion products. Esterone was converted to estradiol-171974 in both normal and inflamed gingiva. The amount of estradiol-171974 found in inflamed tissue was twice as much as that in the normal tissue. The increased conversion of female sex steroids in inflamed gingiva and the nature of their conversion products might be significant factors in the aggravation of gingival inflammation.