MDM2 is the predominant negative regulator of p53 that functions to maintain the appropriate level of expression and activity of this central tumor suppressor. Mdm2-a is a commonly identified splice variant of Mdm2; however, its physiological function is unclear. To gain insight into the activity of MDM2-A and its potential impact on p53, an Mdm2-a transgenic mouse model was generated. Mdm2-a transgenic mice displayed a homozygous-lethal phenotype that could be rescued by a reduction in p53 expression, demonstrating a dependence upon p53. Mdm2-a hemizygous mice exhibited reduced longevity, and enhanced senescence was observed in their salivary glands. In addition, the transgenic mice lacked typical, accelerated aging phenotypes. Growth of transgenic mouse embryonic fibroblasts (MEFs) was inhibited relative to wild-type MEFs, and MDM2-A was shown to bind to full-length MDM2 in an interaction that could increase p53 activity via reduced MDM2 inhibition. Evidence of p53 activation was shown in the Mdm2-a transgenic MEFs, including p53-dependent growth inhibition and elevated expression of the p53 target protein p21. In addition, MDM2-A increased senescence in a p21-independent manner. In conclusion, unexpected roles for MDM2-A in longevity and senescence were identified in a transgenic mouse model, suggesting that Mdm2 splice variants might be determinants of these phenotypes in vivo.