Previously it has been shown that significantly more 2-cell mouse embryos reach the blastocyst stage when cultured in medium supplemented with taurine. In this study, in-vitro fertilized zygotes from a hybrid mouse strain were used to examine the temporal effects of 10 mM taurine on embryonic development in vitro during the preimplantation period. Taurine exerted its beneficial effect exclusively during the first 2 days post-insemination. The effect of taurine on blastocyst formation appeared to be restricted mostly to the period 20-48 h after fertilization, during which time mouse embryos are at the two-cell stage. Although more blastocysts were found when embryos were cultured in taurine-containing medium from 5 to 20 h post-insemination, this difference was not significant compared to the number of blastocysts when embryos were cultured without taurine. Taurine did not appear to affect the two-cell block of mouse embryos from random-bred strains.