The effect of a sleep-dose of etomidate on the plasma concentrations of cortisol, 11-deoxycortisol and 17 alpha OH-progesterone was investigated in eight patients; two other patients received thiopentone. Blood samples were collected at 0, 15 and 30 min after etomidate or thiopentone when an injection of tetracosactrin was given and a final sample was collected at 60 min. The plasma cortisol response to tetracosactrin was depressed below normal in patients given etomidate, and the 11-deoxycortisol response was very high, ranging from 36.4 to 184.7 nmol/l. The two patients that received thiopentone had a normal response to all steroids. The identity of the plasma 11-deoxycortisol in two patients was confirmed by high-performance liquid chromatographic separation and quantification by both spectrophotometric and radioimmunoassay measurements. These results suggest that a single dose of etomidate inhibits 11 beta-hydroxylation of cortisol.