This study determined the ED50and ED90of alfentanil for unconsciousness and anesthesia. A bolus of alfentanil was given to 28 healthy unpremedicated adults undergoing gynecologic or orthopedic procedures in one of four dosages: 100, 150, 200, or 250 μg/kg. Three indicators of induction were assessed 90 s later: eyelid reflex, response to verbal commands to breathe, and response to placement of a nasopharyngeal airway. Succinylcholine, given at 90 s, was followed by tracheal intubation 1 min later.From probit analysis, the ED50and ED90for loss of voice response were 92 and 111 μg/kg, respectively, and for loss of nasopharyngeal airway response, 111 and 169 μg/kg. A high incidence of chest wall rigidity (75%) and movements of the limbs (54%) or eyes (25%) was seen. There were statistically significant increases of the heart rate prior to stimulation and of both the heart rate (21% rise) and systolic blood pressure (10% rise) from control to the peak value following intubation. Differences between alfentanil doses were not significant. Naloxone was required in 36% of patients for end-tidal PCO2greater than 48 mmHg at emergence from anesthesia; no patient required additional naloxone. Nausea or vomiting occurred in 39% of all subjects. Two patients recalled placement of the nasopharyngeal airway.We conclude that alfentanil is an anesthetic, and its ED50(analogous to MAC of inhalational agents) is 111 μg/kg. The blood pressure and heart rate responses to laryngoscopy and intubation were modest after doses that allowed for extubation as early as 51 min after induction.