Microalbuminuria is known to increase in various diseases with potential repercussion on the kidneys and indicates an increase in glomerular intracapillary pressure or changes in permeability characteristics. In this study, we measured whether albumin excretion is affected in patients undergoing anesthesia and surgery, which are both known to induce dramatic changes in renal function and in the release of vasoactive substances such as catecholamines, vasopressin, angiotensin, and prostaglandins. Seven patients with normal renal function and physiological microalbuminuria prior to surgery were studied. Urine samples were collected before anesthesia, just before the begining of surgery, and thereafter 30 min following incision, and 30 min after the end of surgery. Anesthesia induced a significant increase in microalbuminuria, which further increased during surgery. After the end of surgical procedure, microalbuminuria decreased but remained significantly higher than control. This phenomenon may be due to an increase in intracapillary glomerular pressure and/or an alteration in glomerular permeability induced by a direct effect of drugs, or to the action of vasoactive substances on the glomerular structure.