We have investigated the use of alfentanil by infusion to sedate 14 patients during controlled ventilation on the intensive therapy unit (ITU). An initial rate of 24 micrograms.kg-1.h-1 was chosen and altered thereafter according to patient response. Incremental doses of midazolam (2.5-5.0 mg) were given intravenously (i.v.) if indicated. In 4 patients, the use of a muscle relaxant was necessary to allow adequate controlled ventilation of the patient. The mean duration of infusion was 27.9 h (range 10-141 h), and the mean total dose of alfentanil was 69.3 mg (12.5-240 mg). Spontaneous ventilation was rapidly achieved in 11 patients after stopping the infusion. The mean arterial carbon dioxide tension (PaCO2) was 5.38 kPa, 15-30 min after stopping the infusion. The clinical condition of 2 patients necessitated a change in sedation technique and one patient died during the alfentanil infusion. Alfentanil by infusion caused no major cardiovascular effects and did not influence the plasma cortisol response to trauma. There was no major alteration in blood biochemistry or haematology during the infusions of alfentanil. The plasma concentrations of alfentanil during infusion showed a wide variability. These probably relate to both changes in the volume of distribution of the drug and in hepatic clearance.