The cardiovascular effects of nitrous oxide-halo-thane-oxygen anesthesia were studied in eight unpremedicated healthy male volunteers and the results compared with those obtained in previous studies in which only halothane-oxygen anesthesia was used. Adding nitrous oxide to halothane-oxygen anesthesia resulted in less depression of the cardiovascular system than comparable levels of halothane-oxygen anesthesia. The differences were greatest at light levels of anesthesia and early in the anesthetic course. Most of the differences between the nitrous oxide and non-nitrous oxide groups were abolished at deeper levels of halothane anesthesia or after five hours of anesthesia had elapsed. Body temperature increased an average of 0.3 C with increasing depth of halothane anesthesia when nitrous oxide was present. The effect of discontinuing nitrous oxide for 15 minutes was also compared with data from a previous study in which nitrous oxide was added to halothane-oxygen anesthesia. The results seem to conform a sympathetic stimulating action of nitrous oxide.