Isoflurane or halothane was administered at two different inspired concentrations to 21 surgical patients whose average age was 62 years. Most were in physical status (ASA) II or III. Patients were premedicated with diazepam and atropine, anesthesia was induced with thiopental, and tracheal intubation was facilitated with succinylcholine. Respiration was controlled manually or with a ventilator. Anesthesia was maintained with 60 percent N2O and halothane 1 percent, then 0.5 percent, or with N2O-isoflurane 1.2 percent, then 0.6 percent in O2. Variations in the cardiovascular responses among patients given the same anesthetic were as great as the variation in responses between anesthetics. Both produced similar decreases in arterial pressure, cardiac output, and stroke volume. Changes in pulse rate were minimal, and total peripheral resistance changes quite variable, for both drugs. Both halothane and isoflurane appear satisfactory for inhalation anesthesia in the elderly.