Previous magnetic resonance (MR) investigations of middle cerebral artery (MCA) occlusion in rats were limited by the lack of early post-occlusion MR measurements and/or electrophysiological monitoring. Therefore, we have developed a technique which allows to perform MCA occlusion inside the magnet under simultaneous recording of EEG and direct current (DC) potentials for monitoring the ischemic insult. Rats underwent intraluminal thread occlusion of the right MCA inside the MR tomograph via a catheter extension device, while EEG and DC potentials were recorded by non-magnetic graphite electrodes. The thread was slowly advanced until electrophysiological changes appeared. Diffusion-weighted MR images (DWI) were obtained before and repeatedly after MCA occlusion for up to 7 h. Thereafter, rat brains were frozen in situ or fixed by transcardiac perfusion and investigated by biochemical and histological techniques. In 15 of 18 animals (83%), MCA thread insertion caused immediate EEG changes and a negative DC potential shift at 4.4 +/- 1.8 min (mean +/- SD) after occlusion. In all animals with electrophysiological changes, signal intensity of DWI began to increase within the MCA territory at 12-14 min post-occlusion (the end of the first measurement), and continued to rise throughout the observation period. Ischemia was confirmed by demonstrating focal areas of energy depletion on ATP images. In the animals without electrophysiological changes, DWI or biochemical alterations were absent or confined to the central part of caudate-putamen. The histological lesion area of successfully occluded animals amounted to 70.1 +/- 5.8% of the ipsilateral hemisphere at the level of caudate-putamen.(ABSTRACT TRUNCATED AT 250 WORDS)