In uterine endometrium, the level of estradiol is controlled by oxidative 17beta-hydroxysteroid dehydrogenase (17HSD) activity which converts the bioactive hormone to the less active compound estrone. At least three different types of 17HSD (types 2, 4 and 8) use estradiol as their preferred substrate and may contribute to the overall rate of estradiol-inactivation in the uterus. In this study the marmoset monkey (Callithrix jacchus) was used for the investigation of the particular contribution of each type of 17HSD. Northern Blots revealed essentially the same tissue distribution as in the human. Likewise, uterine 17HSD enzyme activity increases in the secretory phase of the reproductive cycle, in parallel to the rise in circulating progesterone levels. Northern analysis of uteri from defined time points of the reproductive cycle showed that only the level of 17HSD2 expression is strongly upregulated in the secretory phase, whereas 17HSD4 and 17HSD8 seem to be expressed constitutively.