We have investigated the effect of nitrous oxide on the middle ear pressure, comparing inhalational anaesthesia with nitrous oxide and halothane and total intravenous anaesthesia with propofol-alfentanil. Fifty patients with normal healthy ears were divided into two groups. In one group (n=25), anaesthesia was induced with thiopentone 6 mg kg(-1), and maintained with halothane 1% and nitrous oxide 66% in oxygen. In the other group (n=25), anaesthesia was induced with alfentanil 25 mu g kg(-1) and propofol 2 mg kg(-1), and maintained with an infusion of alfentanil 10 mu g kg(-1) min(-1) for the first 10 min and then with 0.5 mu g kg(-1) min(-1) and with propofol 10 mg kg(-1) h(-1) for the first 10 min, 8 mg kg(-1) h(-1) for the following 10 min and 6 mg kg(-1) h(-1) thereafter. Patients were ventilated with an oxygen-air mixture (F1O2=0.33). Middle ear pressures were measured during the pre-, intra- and post-anaesthetic period in both ears. A progressive rise was observed (P<0.05) in the first group, whereas values were within the normal limits clinically and there was no statistically significant change in those receiving total intravenous anaesthesia during the intra-anaesthetic period. The time to reach peak pressure with inhalational anaesthesia was 60 min (181.5 mmH(2)O) and to return to normal was 30 min (49.5mmH(2)O) after cessation of nitrous oxide administration. The incidence of nausea and vomiting was less in the patients not receiving nitrous oxide.