Acute and sub-acute inhalation exposure of rats to the organophosphorus compound soman (O-[1,2,2-trimethylpropyl]-methylphosphonofluoridate) reduced the contraction of the bronchial smooth muscle induced by cholinergic stimulation. Acute exposure to 8.51 mg/m3 of soman for 45 min (total dose of 383 mg X min/m3) inhibited the acetylcholinesterase (AChE) activity of the bronchial smooth muscle by 85% and reduced the contraction induced by ACh and carbachol by 70% and 80% respectively. In spite of the extensive inhibition of AChE and reduction in the contraction following cholinergic stimulation, there was no alteration of the binding capacity (Bmax) or the equilibrium dissociation constant (Kd) to [3H]-quinuclidinyl benzilate ([3H]-QNB) in the rat bronchi following such an acute exposure. After sub-acute exposure (40 hr) to 0.45-0.63 mg/m3 of soman (total dose of 1080-1519 mg X min/m3) there was a reduction in AChE-activity of 94% and in the contraction of the bronchial smooth muscle induced by ACh and carbachol of 70%. Furthermore, also a reduction of the binding capacity to [3H]-QNB of approximately 40% was observed. Following exposure to soman by both acute and sub-acute inhalation exposure there was an increase in the apparent affinity (pD2) to ACh in the bronchial smooth muscle, due to the extensive inhibition of the AChE-activity. Inhalation of soman also induced a substantial inhibition of the AChE-activity in the lung (86%), but somewhat smaller inhibition in the hippocampus (70%) and almost no inhibition in the neostriatum (19%). Moreover, it was only in the lung where sub-acute exposure to soman produced a reduction of the binding capacity to [3H]-QNB and the reduction was approximately 50%. The results therefore show that after sub-acute inhalation of a relatively low concentration of the AChE-inhibitor soman, alterations in the number of cholinergic receptors are only observed in the peripheral cholinergic nervous system.