Endotracheal suctioning can elicit bradycardia and hypotension. We compared the effectiveness of nebulized atropine with that of parenterally administered atropine in preventing these responses. Six mechanically ventilated patients who reproducibly experienced a 20% or greater fall in resting heart rate exclusively with endotracheal suctioning were given three trials: a) inhaled atropine sulfate (0.05 mg/kg); b) inhaled saline aerosol (0.05 ml/kg); and c) atropine sulfate (1 mg, given im or iv). Mean heart rate fell from 114 +/- 10 to 45 +/- 5 beat/min with suctioning after the saline aerosol (p less than .001). Both nebulized and parenteral atropine sulfate prevented the bradycardic response in all six subjects (p less than .001). After nebulized saline, three patients also experienced a greater than or equal to 10 mm Hg fall in systolic BP. Both nebulized and parenteral atropine prevented the hypotensive response. Tachycardia occurred after parenteral atropine in all six patients, while only one episode of tachycardia was seen after nebulized atropine. Both parenteral and nebulized atropine can prevent bradycardia and hypotension elicited by endotracheal suctioning, but nebulized atropine may have a wider margin of safety at the doses used in this study.