The effect of enalapril pretreatment on the haemodynamic response to tracheal intubation and surgical stimulation has been studied in 22 patients. Enalapril 5 mg given 4 hours before operation was associated with a significant reduction in the pressor response associated with intubation (p less than 0.05) and surgical stimulation (p less than 0.005) compared with control. Heart rate changes were similar in the two groups. The role of the renin-angiotensin system in relation to the pressor response to sympathetic stimulation is discussed and it is concluded that angiotensin-converting enzyme inhibitors may help improve peri-operative cardiovascular stability.