Magnetic resonance (MR) imaging has recently been used to demonstrate physiological activation of the human brain. This development is of considerable interest to the neurosurgeon planning procedures near brain regions involving specific functions. In the present study, rolandic and visual cortices were imaged with a conventional 1.5-tesla clinical MR imager using a spoiled gradient-recalled acquisition in the steady state sequence. Two patients, one with a right frontal astrocytoma and the other with a left parietal meningioma, underwent MR imaging of rolandic cortex while performing a repetitive finger apposition task. Two patients with complex partial seizures referable to the temporal and occipital regions underwent MR imaging of visual cortex while exposed to repetitive photic stimulation (8.3 Hz). Significant signal intensity changes up to 15% between the activation and rest conditions were observed near the surgical targets at the expected anatomical location of the rolandic and visual cortices. In two of these cases activation measured by MR was compared and found similar to the activation measured at the same plane by H2(15)O positron emission tomography (PET). These results suggest that functional MR and PET techniques can be used to obtain preoperative brain mapping in individual patients considered for neurosurgical procedures.