The effect of immobilization stress (IM-stress) on the muscarinic cholinergic (m-Ch) receptor binding was determined in 8 brain regions using [ 3H]quinuclidinyl benzilate (QNB). IM-stress produced an increase in specific QNB binding in the septum, striatum, hippocampus and pons + medulla oblongata. Scatchard panalyisis revealed that IM-stresss produced an increase in the affinity of m-Ch receptors in the septum, hippocampus andm pons + medulla oblangata, but did not alter the maximum number of binding sites ( B max). In the striatum, an increase in specific QNB binding was due to both the increase in B max and reduction of the dissociation constant ( K d). The present study suggest that IM-stress induces supersensitivity of postsynaptic m-Ch receptors probably due to a deacrease in presynaptic cholinergic activities in the septum, striatum, hippocampus and pons + medulla oblongata. As the m-Ch receptors in the striatum and pons + medulla oblongata. Scatchard analysis revealed that IM-stress produced an increase in the affinity of m-Ch receptors stressful situations in this regions as well as in the septum, hippocampus and cerebral cortex.