Muscarinic cholinoceptor subtypes (M1 and M2) were studied in membrane particles of the rat frontoparietal cortex 72 h and 1, 2, 3, and 4 weeks after ipsilateral lesioning of the nucleus basalis magnocellularis (NBM). The affinity of the ligand used to characterize muscarinic cholinoceptors, 3H-quinuclidinyl benzilate did not significantly change in lesioned compared with sham-operated rats as well as the density of high affinity (M1) sites. Low affinity muscarinic cholinoceptors (M2 sites) were significantly decreased in NBM-lesioned rats 72 h and 1 week after lesioning. The density of M2 sites did not significantly differ in lesioned rats 2 or 3 weeks after NBM lesioning, but increased, in comparison with sham-operation 4 weeks after NBM lesioning. These findings suggest that frontoparietal M2 muscarinic cholinoceptors, which probably have a presynaptic localization, are sensitive to NBM lesions. Their changes at different times after NBM lesioning suggest the occurrence of loss, compensation and upregulation of cholinergic projections arising to the neocortex from the NBM.