Using an immunoblotting technique, we investigated changes in the concentrations of microtubule-associated protein 2, 200-kDa neurofilament, tubulin, myelin-associated glycoprotein, and 2':3'-cyclic nucleotide 3'-phosphodiesterase in the brains of 40 rats following occlusion of the left middle cerebral artery or sham operation. Compared with those 4 hours after surgery, concentrations of all proteins decreased significantly in the left hemisphere 3 days after surgery (p less than 0.01). Microtubule-associated protein 2 was the most susceptible to ischemia, and its mean +/- SEM concentration decreased to 23 +/- 9.4% of that in concurrent sham-operated controls. Degradation products of microtubule-associated protein 2 and myelin-associated glycoprotein were detected on the blots. Furthermore, in the contralateral hemisphere (where calpain might be activated), concentrations of these two proteins decreased to 57 +/- 12.0% and 83 +/- 4.3% of those in concurrent sham-operated controls, respectively, 3 days after surgery. Changes in the concentrations of cerebral proteins in the contralateral hemisphere are important for understanding clinical symptoms not attributable solely to the ipsilateral lesion following a focal cerebral stroke.