The haemodynamic effects of equi-anaesthetic doses of alphaprodine and sulfentanil were evaluated in mongrel dogs ventilated mechanically. The effects of the addition of atropine were assessed for each drug, as were the effects of breathing nitrous oxide 60 per cent with oxygen and nitrogen 60 per cent with oxygen and the administration of naloxone. The results of this study demonstrate that anaesthetic doses of sulfentanil produce little change in cardiovascular dynamics in atropinized dogs and only small changes in dogs without atropine premedication, irrespective of the infusion rate. In contrast alphaprodine causes a significant cardiovascular depression irrespective of the presence or absence of atropine premedication. Naloxone reversed the anaesthetic effects in all dogs anaesthetized with alphaprodine but did not reverse the anaesthetic state in any dog given sulfentanil. Our data suggest that sulfentanil deserves evaluation as a narcotic anaesthetic in man.