Nitrogen has recognized narcotic potential at hyperbaric pressures. No narcotic effect of helium has been demonstrated at any pressure. We evaluated the effect of nitrogen in air at one atmosphere on human performance by comparing it with helium-oxygen using a four-alternative divided-attention task that requires rapid response to auditory and visual signal changes. There was a 9.3 per cent decrease in response time when subjects breathed helium-oxygen, a signigicant change (P less than 0.001). This change could not be ascribed to practice since the order of presentation of gases did not have a significant effect. It concluded that the nitrogen in ambient air slightly but measurable impairs human performance compared with a non-anesthetic gas such as helium.