The efficacy of fentanyl, 30 micrograms/kg, was evaluated as an anesthetic induction agent in 72 ASA I-III patients scheduled for 2-4-hr operations. The effect of preinduction pretreatment with pancuronium and/or diazepam and the incidence of loss of consciousness (anesthesia), recall, rigidity, abnormal muscle movements, and hemodynamic changes were documented. Seventy-four percent of all patients became anesthetized. Diazepam pretreatment enhanced but did not ensure success of anesthetic induction. There was a significant correlation between age and the incidence of unconsciousness (P = 0.0287) and all patients over 60 yr old were anesthetized with 30 micrograms/kg of fentanyl. The incidence and severity of rigidity was reduced by pancuronium (P = 0.0002) but not by diazepam pretreatment. However, pancuronium plus diazepam produced a significant reduction in the incidence of rigidity when compared to pancuronium alone (P = 0.031). A significant positive correlation between age and the incidence of rigidity (P = 0.003) was found. Six patients had focal and one patient global tonic-clonic abnormal muscle movements. Diazepam but not pancuronium significantly decreased both heart rate (P = 0.05) and blood pressure (P = 0.04). Seventeen patients required reversal of narcotic effect to restore adequate spontaneous respiration after surgery. No patient required postoperative mechanical ventilatory assistance. The results of this study demonstrate that 30 micrograms/kg of fentanyl is not a reliable anesthetic induction dose in patients less than 60 yr old. Both age and premedication enhance the anesthetic capabilities of induction with fentanyl.(ABSTRACT TRUNCATED AT 250 WORDS)