The authors compared the respiratory effects of an intravenous infusion of ketamine (1 mg · kg-1) with droperidol (0.1 mg · kg-1), or placebo on three different occasions in a double-blind, randomized fashion in eight healthy volunteers. Breathing pattern, thoraco–abdominal motion, end-expiratory positions of the rib cage and abdomen, arterial hemoglobin oxygen saturation (SaO2), and end-tidal carbon dioxide concentration (FECO2) were continuously measured with noninvasive techniques. During the 1-h monitoring period following drug injection, droperidol produced occasionally significant but clinically unimportant differences in respiratory variables when compared with placebo. In contrast, ketamine induced a significant (P< 0.001) and persistent increase in minute ventilation (+75%) from 5 to 20 min after start of infusion by increasing both the driving (i.e., tidal volume/inspiratory time [VT/T]) and the timing (i.e., in-spiratory time/total respiratory cycle time [T/T]) components of ventilation (Milic-Emili J, Grunstein MM: Chest 70 (Suppl): 131–133, 1976). This was obtained without any significant change in end-expiratory positions or change in relative rib cage contribution to tidal volume. Despite multiple apneic episodes observed with ketamine, the subjects maintained a stable SaO2and FECO2, indicating no resting respiratory depression. This study, performed with a noninvasive respiratory monitoring technique, confirms that droperidol infused over 5 min at a clinically used dosage does not cause respiratory depression in healthy subjects, whereas ketamine produces an important ventilatory stimulation.