Renin and angiotensin II have been measured before and 10 minutes after anaesthesia, in 23 patients undergoing minor gynaecological surgery. Twelve were anaesthetised with ketamine 2 mg/kg i.v. and the remainder with thiopentone 5-10 mg/kg i.v. Arterial blood pressure was monitored automatically throughout. The arterial blood pressure rose significantly in the group given ketamine and plasma AII concentration fell. In the group given thiopentone there was no significant overall change in blood pressure but an increase in AII. Plasma renin concentration and activity showed significant increases following the administration of ketamine but were unchanged following thiopentone. Overall, there was a significant inverse relationship between the change in blood pressure and the change in AII. This is compatible with a negative feedback being exerted on the renin-angiotensin system by the raised blood pressure, which has itself been evoked by some quite different factor or factors. We conclude that the renin-angiotensin system is not concerned in the pressor response to ketamine.