The MR appearance of acute subarachnoid hemorrhage experimentally produced in Macaca monkeys and observed in patients with clinically documented acute subarachnoid hemorrhage is presented. Subarachnoid hemorrhages were produced in two Macaca Nemestrema monkeys using the technique of Frazee. CT and MR imaging were performed immediately after the procedure and at frequent intervals up to two week post hemorrhage. MR including T1 and T2 weighted multiplanar spin echo images were obtained. The imaging studies were compared with clinical evaluations and pathological specimens of all animals. Findings in the experimental animals are correlated with those observed in patients with clinically documented subarachnoid hemorrhage. The results show that acute subarachnoid hemorrhage (SAH) can be detected with MRI as isointense signal replacing normally black CSF spaces on T1 weighted images. Signal changes most likely relate to protein water binding associated with the clotting mechanism rather than oxidative denaturation of hemoglobin. Imaging performed experimentally and clinically beyond four days, however, showed a marked increase in signal intensity on T1 weighted images which probably does result from methemoglobin formation within the clot matrix. Although CT remains the gold standard in detecting acute SAH, MR does provide some sensitivity to its presence.