Cortical activity related to the late component of the cutanomuscular reflex was studied by measuring somatosensory evoked magnetic fields (SEFs) during isometric contraction of the reflex-induced muscle. After electrical stimulation to the index finger with intensity of three times of the perception threshold, three peaks of the cortical activity were detected within the somato-sensory area contralaterally to the stimulation site. Three dimentional location and the amplitude of the equivalent current dipoles for each peak were compared to that of observed SEF without muscle contraction. Significantly increased third component of SEFs (the latency of about 50 ms) always preceeded ca 20 ms against the peak of the late component of the cutaneomusucular reflex observed in the first dorsal interosseous (1DI) muscle. The conduction delay from the primary somatomotor cortex to the 1DI was determined as also ca 20 ms according to the result of the transcranial magnetic stimulation. We conclude from these evidences that all or part of the ingredient of the third component of SEFs may contribute to inducement of the late excitatory component of the cutaneomuscular reflex.