Ten male volunteers performed 3 tasks before, during, and following administration of 3 levels of alveolar halothane (0.05, 0.1, and 0.2%) for one-half hour each and (on a separate occasion) enflurane (0.15, 0.3, and 0.4%) for one-half hour each. The concentrations of halothane tested representing 7, 13, and 27% of MAC values, respectively; those of enflurane were 9, 18, and 24% of MAC. The tasks were a choice-reaction time test, a digit span test, and a Purdue Pegboard Assembly test. Volunteers also were tested for amnesia with word pairs and a picture. No effects or only slight effects on mental function could be detected at the lowest concentrations of either agent. At higher concentrations both agents impaired function, as indicated by an increase in reaction time, decreased ability to remember numbers, and decreased ability to assemble a simple structural array. Amnesia for word pairs but not pictures occurred at 27% of MAC for halothane and 18% of MAC for enflurane. All test scores reverted to control levels within one-half hour after discontinuing the anesthetic. Our results indicate that subanesthetic but not trace levels of enflurane or halothane can impair mental performance and manual dexterity and produce amnesia. The levels of anesthetic required far exceed those experienced by operating room personnel. However, such levels might be found for several hours in patients following prolonged anesthesia.