Plasma fentanyl concentrations were measured during and after transdermal fentanyl delivery in groups of patients undergoing general surgery. At 8 and 12 h, concentrations did not differ from those observed in a matched group of patients receiving fentanyl by i.v. infusion. At 24 h, concentrations were significantly lower in one of the transdermal groups. Plasma fentanyl clearance did not differ significantly between the groups. Plasma fentanyl concentrations decreased slowly after removal of the transdermal system.