The authors investigated the hemodynamic, metabolic, electroencephalographic (EEG), and electromyographic (EMG) characteristics of narcotic-induced rigidity during induction of anesthesia with alfentanil (175 micrograms/kg) in 10 patients. Thiopental (4 mg/kg) was administered to a ten-patient control group. Rigidity was quantified in eight muscle groups (sternocleidomastoid, deltoid, biceps, forearm flexors, intercostal, rectus abdominus, vastus medialis/lateralis, and gastrocnemius). Marked rigidity was observed in all muscle groups in all patients receiving alfentanil and in none receiving thiopental. Central venous pressure increased with onset of rigidity, while mean arterial pressure and cardiac index remained unchanged. Manual ventilation was extremely difficult during alfentanil-induced rigidity. Arterial oxygen tension decreased more rapidly during rigidity than during the same time interval in the control group, while patients experiencing rigidity were more acidotic, as reflected by greater increases in base deficit. The EEG demonstrated an anesthetic state without seizure activity. The immediate increase in central venous pressure with the onset of rigidity, along with occasional simultaneous parallel variations in central venous pressure and the EMG, strongly suggest a mechanical mechanism for the change in central venous pressure. The metabolic changes during rigidity may be partly related to the absence of the normal cardiovascular reflexes that are reported to occur during voluntary isometric muscle contractions. A neurochemical mechanism of narcotic-induced rigidity is briefly reviewed.