Previous work on the early development of the golden hamster includes the investigation of Ochs (1908), Graves (1945) and Ward (1948), all at the level of the light microscope. Austin (1963) has examined the ultrastructure of the oocytes of the golden hamster while Enders & Schlafke (1965) have observed the pre-implantation stages of pregnancy. The ultrastructure of implantation has been studied in two other species of myomorph rodents: the mouse (Potts, 1966a; Reinius, 1967) and the rat (Enders & Schlafke, 1967). Implantation is taken as beginning when the zona pellucida is lost and the trophoblast is in contact with the uterine epithelium throughout its circumference. This takes place at between 80-100 hpost coitum. Previous studies have been made on specimens embedded in paraffin, and the shrinkage which occurs with this method of preservation has caused implantation to appear to begin considerably later than this: Graves (1945) gives it as beginning at 5 days, Ward (1948) as 4 days 8 h. Implantation is antimesometrial and the embryonic pole is orientated towards the mesometrium.