When two human volunteers were imaged with magnetic field gradient dB/dt of 61 T/s RMS, the subjects reported, to our surprise, feeling muscular twitches synchronous with gradient pulses over repeated experiments. No adverse or sustained effects were seen. Experiments in a canine, intended to assess the safety of MR imaging with dB/dt of up to 66 T/s RMS, failed to induce detectable changes in the electrocardiogram or to show any signs of gross response to gradient pulsing. Although these data are preliminary, and largely anecdotal, they suggest a level above which such stimulation may occur. We believe that this is the first report of direct human stimulation in an MRI device and that determination of the stimulation threshold may have impact on the selection of appropriate operating points for magnetic imaging systems. © 1990 Academic Press, Inc.