Participation of the primary motor cortex (M1) in motor imagery was addressed using functional magnetic resonance imaging at 2.0 T and 2 x 2 x 4 mm3 resolution in six right-handed subjects. Paradigms comprised visually cued execution and imagination of a sequential finger-to-thumb opposition task (12 s) contrasted with motor rest and visual imagery (18 s), respectively. Motor execution activated M1 as well as other parts of the motor system including supplementary motor area (SMA) and premotor areas (PM). In contrast, motor imagery did not lead to activations in M1 except for 1/6 subjects but involved SMA and PM bilaterally as well as the anterior intraparietal cortex. Moreover, a region-of-interest analysis revealed a weak initial MRI signal increase in M1 in 4/6 subjects. This novel finding of a transient response reflecting the onset of imagination which does not lead to sustained M1 activation may explain previous contradictory reports.