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Previous investigations have demonstrated that weak artificial magnetic fields can inhibit nocturnal pineal melatonin synthesis in rats. Nuclear magnetic resonance imaging (MRI) is an increasingly popular procedure used for advanced medical imaging and requires patients to be exposed to a variety of magnetic field conditions which could alter nocturnal melatonin levels. In the present study four healthy adult male subjects were exposed to a typical clinical MRI procedure for 40.5 min during the mid-dark portion of their daily light-dark cycle. Serum levels of melatonin, cortisol, prolactin and growth hormone were measured at hourly intervals for 3 hrs prior to, and for 3 hrs after, exposure to the MRI procedure or a sham exposure condition. Analysis of the results failed to show any significant changes in any of the hormone levels as a result of exposure to the MRI procedure. These preliminary results fail to provide any evidence for possible adverse effects of exposure to MRI in humans.