This double-blind, randomized trial compared the onset of sedation with two patient-controlled sedation regimens, allowing a maximum of 16 or 25 mg min−1 propofol. Forty fit young patients presenting for elective surgery were asked to try to put themselves to sleep using the system. Onset times of sedative effect, slurred speech and amnesia were recorded. All patients achieved satisfactory sedation, and none became oversedated. Patients receiving 16 mg min−1 propofol were not reliably sedated within 5 min and took significantly longer to develop slurred speech and amnesia (P< 0.01 for both). We conclude that this maximum infusion rate does not produce amnesia or sedation rapidly enough to be clinically useful. A maximum infusion rate of 25 mg min−1 allowed rapid sedation in all patients without oversedation and may be an acceptable compromise between efficacy and safety.