1. Electroencephalographic activity was registered in healthy subjects before, during and 90 minutes after intravenous injections of sufentanil and fentanyl. 2. With a dosage of ratio of 1:10, i.e., 0.025 mg/70 kg body weight sufentanil: 0.25 mg/70 kg body weight fentanyl as well as 0.015 mg/70 kg body weight sufentanil: 0.15 mg/70 kg body weight fentanyl, sufentanil was hypnotically more potent from fentanyl. 3. Sufentanil acts more powerfully on the respiratory regulation than does fentanyl. Periods of apnoea occur more frequently but can be interrupted by external psychophysiological stimulation. 4. The evaluation of indices of certain EEG-waves, characterizing well defined electroencephalographic stages corresponding to the waking-sleeping-behaviour confirmed the visual over-all analysis of the EEG-stages. 5. Physiological observations on heartrate, systemic blood pressure, rapid eye movements during wakefulness, and slow eye movements during sleepiness showed no reliable differences between sufentanil and fentanil. The systematically documented symptoms and vegetative signs of subjects also showed neither differences nor side-effects.