The effects of magnesium sulfate and nifedipine on regional cerebral blood flow were compared after ligation of the middle cerebral artery in rats. Twenty-one rats were divided into 3 groups of 7 each. The groups were the magnesium group, the nifedipine group, and the control group. Animals were anesthetized with pentobarbital. The middle cerebral artery was ligated, and magnesium sulfate was infused at a rate of 16 mg/kg/min for 30 min in the magnesium group, and nifedipine was infused at a rate of 10 micrograms/kg/min for 45 min in the nifedipine group. Using [14C]-iodoantipyrine, regional cerebral blood flow was measured 1 hr after middle cerebral artery ligation in the ischemic cortex, contralateral cortex, thalamus, hypothalamus, pons, medulla and cerebellum. In the control group, the mean cerebral blood flow was 30.7 +/- 12.2 ml/min/100 g in the middle cerebral artery-ligated cortex, and 75.9 +/- 10.5 ml/min/100 g in the contralateral cortex. Nifedipine did not significantly alter the cerebral blood flow in any of the brain regions studied. However, in the magnesium group, the regional cerebral blood flow in the middle cerebral artery-ligated cortex was significantly higher (64.2 +/- 8.8 ml/min/100 g) than that recorded in the control and nifedipine groups, whereas the regional cerebral blood flow in the contralateral cortex was not significantly different from other groups. Our data showed that magnesium was effective and superior to nifedipine in improving blood flow in the ischemic area of the brain.