Abstract. We report the first successful culture of 8-cell/morula stage rat embryos in a fully synthetic medium supplemented with 3% crystalline bovine serum albumin. Eighty-four per cent of morulae developed to blastocysts, showing that this is a highly efficient culture medium for in vitro studies on rat pre-implantation embryos. Blastocyst formation was severely inhibited by antioestrogen (nafoxidine 3 μg/ml) but no further reversal was obtained by giving oestrogen to culture medium containing this antagonist.