The aim of this study was to evaluate the rat periodontal ligament cell viability under 0 o C/2 MPa condition up to one week using in vivo 3-(4,5-dimethylthiazol-2-y1)-2,5-diphenyltetrazolium bromide (MTT) assay.Materials and Methods: As soon as 110 upper molar teeth of rats were extracted, they were stored in Hartman's solution under 0 o C/2 MPa condition for 1, 2, 3, 4, and 7 days each. All specimens were treated with in vivo MTT assay and the value of optical density was measured by ELISA reader. These values were statistically analyzed by one-way ANOVA.Result: There was no statistical difference on MTT value between immediate and 1 day storage group. There were statistically significant differences between 1 day and 2 days storage, 2 and 3 days storage groups, respectively. Teeth of 3, 4, and 7 days storage groups showed significantly lower MTT values compared with shorter period storage groups. Conclusion:When the MTT values were substituted in standard curve, 1 day storage group at 0 o C/2 MPa condition showed 68% cell viability when compared with immediate group. It dropped to 13% at 2 days, and to less than 5% at 3 days or more.