The aim of the present study was to examine effects of altering thawing conditions and procedure of addition and dilution of Me2SO on the viability of frozen-thawed rabbit morulae. Five hundred and sixty two rabbit morulae were cooled from room temperature to -80 degrees C at 1 degree C/min in the presence of 1.5 M dimethyl sulfoxide (Me2SO) using a programmable liquid nitrogen vapor freezing machine with an automatic seeding device, cooled rapidly, and stored in liquid nitrogen. When Me2SO was added in a single step, the frozen embryos were thawed in ambient air at 40 degrees C/min and Me2SO was diluted in a single step, 99 of 107 (93%) embryos cultured for 48 hr and 12 of 32 (38%) embryos transferred to 6 recipients developed to expanding blastocysts and viable fetuses, respectively. When Me2SO was added in a single step and the frozen embryos were thawed at the same rate and transferred directly without removal of Me2SO to culture media or oviducts of 8 recipients, 67 of 75 (89%) embryos cultured and 12 of 40 (30%) embryos transferred developed to expanding blastocysts and viable fetuses, respectively. There were no significant differences between these survival rates and survival rates obtained by conventional method, i.e., frozen embryos were thawed at 4 degrees C/min by interrupted slow method and Me2SO was added and diluted in a stepwise manner.