In different brain regions of the rat we studied the effect of chronic feeding with the organochlorine insecticides p,p'-DDT and gamma-HCH on the cholinergic muscarinic receptors. Using [3H]quinuclidinyl benzylate binding to membranes from cerebral cortex, medulla pons, diencephalon, and cerebellum it was found that the two insecticides produced a decrease in the number of muscarinic receptor sites in cerebellum; while gamma-HCH also reduced these receptors in diencephalon. In both cases no changes in receptor affinity were observed. It is suggested that the chronic treatment with these organochlorine insecticides may cause an alteration in cholinergic transmission leading to a down regulation of the muscarinic receptor in certain brain regions.