The present study was undertaken to examine the effect of a cooling regimen during vitrification on survival and morphological appearance of human oocytes after warming and the developmental potential of day 3 embryos. Aged human oocytes that had failed to fertilize and human embryos derived from abnormally fertilized zygotes that showed one pronucleus or three or more pronuclei were used as models in this study. In the first part of the study, 928 aged human oocytes that had failed to fertilize were vitrified. The viability of oocytes after vitrification using the hemi-straw system was slightly higher than it was using the cryoloop after warming (85.4% (410 of 480) versus 80.6% (361 of 448)), but the difference was not significant. In the second part of the study, 266 embryos were vitrified. The survival of day 3 embryos after vitrification was improved by using a mixture of ethylene glycol and dimethyl sulphoxide in combination with the hemi-straw system method rather than the cryoloop method (89.7% (122 of 136) versus 83.8% (109 of 130)), but the difference was again not significant. The potential for development up to compaction on day 4 in embryos vitrified using the hemi-straw system method was significantly higher (37.7%) than it was in embryos vitrified using the cryoloop (29.4%; chi(2), P = 0.002). The hemi-straw system and cryoloop methods of vitrification are both successful, easy to perform, and demonstrate the ability of both carriers to vitrify different stages of development (oocytes and day 3 embryos). In addition, the success of both methods appears to be related to the rate of cooling and the lower concentration of cryoprotectant used.