The effect of exposure to the magnetic and radio-frequency fields associated with Nuclear Magnetic Resonance Imaging (MRI) on survivability and long-term stress reactivity levels was examined in male rats. Rats in the experimental condition were exposed to MRI for 22.5 minutes for five successive days (Expt. 1) or 23.3 minutes for twenty-one successive days (Expt. 2). Sham field exposed animals received the identical treatment as the exposed animals except that the magnetic and radio-frequency fields were absent. Control rats were also maintained. Thirteen to twenty-two months after the exposure procedure the remaining rats were sacrificed and their whole body, spleen, heart, thymus, and adrenal weights were recorded. Blood samples were taken for measurements of red and white blood cell counts, hemoglobin content, as well as adrenocorticotrophin and corticosterone levels. The results fail to provide any evidence for changes in survivability and long-term stress reactivity levels in rats exposed to MRI and thus give no grounds to challenge the view that MRI is a safe diagnostic procedure.