The in-vivo development of murine morula stage embryos frozen-thawed once or twice and embryos biopsied after one freezing cycle and refrozen was studied. Embryos (n = 860) were cryopreserved using a rapid-freezing procedure. At least 24 h after freezing, embryos were thawed and cultured in vitro for 3 h. In experiment I, morphologically intact embryos were either transferred (n = 180) into recipients or refrozen (n = 160). Unfrozen embryos (control group, n = 180) and refrozen embryos stored for at least 24 h and then thawed, were transferred into recipients. In experiment II, embryos frozen once were thawed and biopsied or sham-biopsied (n = 230 and 180 respectively) and refrozen (n = 226 and 179 respectively). They were thawed and transferred (n = 192 and 160 respectively) into recipients. Recipient mice were either killed on day 15 after embryo transfer and number of implantation sites and live fetuses recorded or pregnant recipients (n = 6, experiment II) were allowed to carry the fetuses to term. There was no difference in the survival rate of embryos at thawing between those frozen once or twice (91 versus 93%). The implantation rate and number of live fetuses in the pregnant recipients at necropsy among those transferred with unfrozen embryos (57% and 51%; 8/9), embryos frozen once (55% and 45%; 8/9) or twice (51% and 48%; 6/8) was not different. There was no difference in the survival rate of refrozen embryos biopsied or sham-biopsied after one freezing cycle (89 versus 87%). The implantation rate and number of live fetuses in pregnant animals transferred with biopsied or sham-biopsied embryos was not different (64 and 41% versus 57 and 37% respectively). All six pregnant animals allowed to carry the fetuses to term delivered normal live fetuses (n = 39). On mating 12 females with six males of the progeny born out of biopsied embryos, all became pregnant and delivered live fetuses. It may be concluded that murine biopsied and intact embryos can be successfully refrozen by rapid-freezing procedure.