Previous attempts to obtain digynic triploid mouse development in vivo have either been entirely or only marginally successful, generally with the production of heteroploid rather than triploid conceptuses. We report that when a single intraperitoneal injection of 15 pg of cytochalasin D is given to recently mated female mice during a restricted period following ovulation induced by exogenous gonadotrophins, between 14 and 18% of conceptuses isolated on the 10th day of gestation had a triploid chromosome constitution. Triploidy was only induced in those eggs that were exposed to cytochalasin D when they were passing through a critical phase of the second meiotic division corresponding to the time when the second polar body was about to be extruded. Exposure to this agent either before or after this critical period only results in the development of normal diploid conceptuses. When females were mated to males carrying an easily recognisable paternally derived 'marker' chromosome, convincing cytogenetic evidence was obtained that only digynic triploidy was induced. No examples of diandric triploidy were recognised when conceptuses were analysed on the 10th day of gestation. The technique described therefore represents a simple and direct means of inducing digynic triploid mouse conceptuses whose development potential may be compared directly with that of their normal diploid littermates.