Recent studies have shown that many organophosphates can bind competitively and noncompetitively to membrane muscarinic receptors. The present study investigated the responses of the rat aortic rings to diisopropyl-flurophosphate (DFP), an organophsophorus cholinesterase inhibitor, and the possible involvement of muscarinic receptors. DFP caused a concentration-dependent contraction when added cumulatively from 10(-8) to 10(-4) M. This contraction was inhibited in a noncompetitive manner by high concentrations of atropine (1.5 x 10(-6) and 1.8 x 10(-6) M) but was unaffected by similar concentrations of selective muscarinic receptor subtype antagonists, pirenzepine, 11-2[2-[(diethylamino)methyl]-1-piperidinyl]acetyl-5, 11-dihydro-6H-rido[2,3-b][1,4]benzodiazepin-6-one (AF-DX116) and 4-Diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP). Phentolamine, an alpha-adrenoceptor antagonist, was able to inhibit the DFP-induced contraction in a noncompetitive manner at a concentration of 10(-7) M. These findings suggested that the DFP-induced contraction in the rat aortic rings was mediated by norepinephrine that was released from sympathetic nerve terminals present in the aortic rings.