A method for culturing rat embryos between days 14 and 15 has been developed. On gestation day 14 (morning of sperm and plug = day 0), laparotomies were performed on time-mated rats that had been anaesthetized with halothane. The embryos, with open attached yolk sacs and placentas, were cultured in 650-ml plastic flasks with 120 ml medium (0.7% bovine serum albumin in Earle's balanced salt solution with antibiotics, prewarmed to 37 degrees C and continuously flushed with water-saturated 95% O(2)/5% CO(2), pH 7.4) in a Dubnoff metabolic shaker with gentle agitation. Viability and morphology were assessed after 26 hr in culture. On day 15, embryonic growth and development were similar to that in in vivo controls, and viability was high (55%).