AbstractAcetylcholinesterase (AChE) activity was measured in bronchial tissue homogenate and blood from rats subjected to single and repeated 4‐h daily inhalation exposure to toluene diisocyanate (TDI) or control atmospheres. A single 4‐h exposure to TDI in the concentration range of 0.7–4.3 ppm did not decrease AChE activity in bronchial tissue but a 4‐h exposure to 0.6 ppm TDI, or greater, for two consecutive days did reduce this activity (19 to 33 of the controls). Increasing the level of exposure to TDI for two consecutive days from 0.6 to 4.0 ppm and extending the length of exposure to 1.2 ppm TDI from 2 to 4, 9 or 14 days produced no further decrease in bronchial ACHE activity. Throughout these experiments, blood AChE activity remained unchanged. In rats exposed to 0.3 or 1 ppm TDI for 3 weeks, staining of the bronchial smooth muscle for AChE was reduced (36 of the controls) after exposure to 1 ppm TDI. These results support, for the first time,in vivoand localized anti‐AChE activ
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