Department of Veterinary Anatomy, University of Bristol, Bristol, BS1 5LS (Received 22 May 1974) Hydroxysteroid dehydrogenases (HSD's) acting at the 3α, 3β and 16β positions have been demonstrated histochemically in the trophoblast of the established placenta of the pig and 17β-HSD has been identified in the maternal placental epithelium (Christie, 1968; Dufour & Raeside, 1969). This 17β-HSD is also found in the uterine epithelium of the interlocular and unoccupied parts of the pregnant uterus but is absent from the non-pregnant uterus (Flood, unpublished). The object of the present study was to determine the time of appearance of these enzymes. Eight gilts and one sow were killed at 10, 12(2), 13(2), 14, 15, 16 and 19 days of gestation. The day after first service was regarded as day 0. The uteri were removed within 10 min of slaughter and in the case of the gilt killed at 10 days' gestation, ten