In animal models authors have dealt with the question of whether hypotension alone can cause an acute-phase response even in the absence of marked blood loss and trauma. Sodium nitroprusside (SNP), which was employed in the animal models, is also used to induce hypotension in humans. Since no data are available on human subjects plasma concentrations of interleukin-6 (IL-6), an important mediator of the acute-phase response, were studied in patients during SNP infusion for induction of hypotension. METHODS. After approval by the local ethics committee, 20 patients scheduled for elective oto-rhino-laryngological operations participated in this randomised prospective study. Anaesthesia was induced with fentanyl, etomidate, vecuronium and succinylcholine and was maintained with isoflurane in 66% N2O and 33% O2. Ten patients received SNP to reduce mean arterial blood pressure to 50 mmHg, while another ten patients served as controls. Blood samples were taken before the induction of anaesthesia, during surgery (at the end of the SNP infusion), 60 min after surgery and on the day after surgery. IL-6 concentrations were determined by means of enzyme-linked immunosorbent assay. Epinephrine and norepinephrine in plasma were measured by high-pressure liquid chromatography with electrochemical detection. RESULTS. The IL-6 plasma concentration increased significantly from 3.2 (0-7.5) pg/ml (median and range) to 31.8 (9-42.2) pg/ml in the SNP group and from 3.5 (0-8.3) pg/ml to 15.2 (7.4-19) pg/ml in the control group on the morning after surgery. The IL-6 values at this time were significantly (P < 0.05) higher in the SNP group than in the controls. Norepinephrine increased significantly from 263 (150-920) pg/ml (median and range) preoperatively to 419 (115-897) pg/ml, and the epinephrine concentrations rose significantly from 77 (12-159) pg/ml to 115 (83-330) pg/ml at the end of SNP administration. No significant changes in the catecholamine concentrations were observed in the control group. CONCLUSIONS. The SNP infusion exerted an important additional stimulus for IL-6 release after relatively mild surgical trauma in both groups. This finding is probably due to the liberation of NO from the SNP molecule and an increase in the intracellular concentration of cGMP. The elevation of the plasma catecholamines immediately after SNP administration should also be taken into account, because an augmentation of the cAMP in various cell types has been proven to result in increased release of IL-6.