Various methods have been developed in recent years for growing post\x=req-\ implantation rat and mouse embryos in culture. Young embryos\p=m-\upto early somite stages\p=m-\growwell in static medium, e.g. simple watch-glass cultures (New, 1966; Steele, 1972). Older embryos require flowing medium, as in the culture systems of New (1967), Tamarin & Jones (1968), Robkin, Shepard & Tanimura (1972) and Cockroft (1973). These methods and some of their applications have been reviewed by New (1973). Recently, a simpler technique for maintaining embryos in flowing medium has proved useful in this laboratory (New & Brent, 1972; New & Mizell, 1972). It consists of the culture of free-floating embryos in a constantly rotated tube which contains serum and an appropriate gas phase. We give here a short description of the method and a comparison of the results with those of the `circulator' method (New, 1967). Embryos, together with the placentae and embryonic membranes, are removed from the pregnant uterus and dissected from the surrounding decidua