To determine the contribution of presynaptic cholinergic mechanisms to the increased sensitivity of a genetically selected line of Sprague-Dawley rats (Flinders S-line) to the anticholinesterase, diisopropyl fluorophosphate (DFP), rats were sacrificed by focused microwave irradiation of the head 1 min after a pulse injection of deuterium-labeled choline into the tail vein. The S-line rats exhibited higher concentrations of labeled acetylcholine (ACh) in the cortex than the rats bred for resistance to DFP (Flinders R-line). To determine the contribution of postsynaptic cholinergic mechanisms the concentration of brain muscarinic ACh receptors (mAChR) was determined. The S-line rats exhibited higher concentrations of striatal and hippocampal mAChR than the R-line rats. Thus, both pre- and postsynaptic cholinergic mechanisms may contribute to the increased sensitivity to DFP but their relative importance varies with brain region: increased ACh synthesis in the cortex and increased concentrations of mAChR in the striatum and hippocampus.