the purpose of our study was to determine where organophosphates act in the brain to cause respiratory depression. our focus was on the chemosensitive sites on the ventral surface of the medulla of anesthetized cats. for this purpose, drugs were applied bilaterally on the ventral surface of the medulla, specifically at the rostral, intermediate and caudal areas using cottonoid pledgets, while monitoring tidal volume and respiratory rate. doses of 6.25, 12.5 and 25.0 mu sub g/side of diisopropylfluorophosphate (dfp) applied to the intermediate area produced dose-related increases in tidal volume. in some cases the highest dose tested also resulted in respiratory depression that was characterized by a slowing in respiratory rate only minor effects on respiration were observed upon bilateral application of dfp to rostral and caudal areas. topical application to atropine to the intermediate area counteracted the respiratory effects of both dfp and soman. furthermore, topical application of oxotremorine, a muscarinic receptor agonist drug, to the intermediate area produced effects similar to both dfp and soman. these results indicate that the cholinergic receptor responsible for mediating the respiratory stimulant and depressant effects of anticholinesterase agents is the muscarinic receptor. (sdw)