Total intravenous anesthesia can be driven by a computer using bispectral index as a controller. Such a system matches anesthetic agent requirements to the needs of each individuum and thus allows an unbiased evaluation of the influence of nitrous oxide. Two groups of patients are compared: one ventilated with a oxygen-air mixture and one with oxygen-nitrous oxide mixture. Anesthesia is provided by a "dual-loop" which adapts propofol and remifentanil target concentrations to maintain a bispectral index between 40 and 60 in both groups.