This study examined the influence of mode of ventilation (spontaneous or controlled) on the target blood concentrations required to maintain anaesthesia with 'Diprifusor' (a target controlled infusion system for propofol) in 40 healthy, unpremedicated, adult patients undergoing knee arthroscopy. All patients were given alfentanil (10 μg.kg−1) and ketorolac (10 mg) immediately before induction and all received a 2:1 mixture of nitrous oxide:oxygen. An initial target blood concentration of propofol of 6.0 μg.ml−1 was used in most patients to induce anaesthesia. The blood target concentration required to produce acceptable anaesthetic conditions was not significantly influenced by the mode of ventilation. The mean maintenance target concentration for propofol was 3.9 (SD 0.83) μg.ml−1 in the ventilated group and 3.5 (SD 0.82) μg.ml−1 in the group of patients breathing spontaneously.