Interleukin-6 (IL-6), a cytokine involved in the pathogenesis of sepsis and septic shock, and lymphocyte subpopulations were measured in blood circulation of patients receiving sodium nitroprusside (SNP) for induction of hypotension. The aim of this study was to evaluate whether this procedure influences distribution of lymphocyte subsets and IL-6 response. 30 patients of ASA physical status I and II scheduled for nose-septum correction were randomly assigned to the SNP- or control group (without SNP). Patients were anaesthetized with fentanyl, etomidate and isoflurane in 66% nitrous oxide. SNP was administered continuously during 60 min and mean arterial blood pressure was reduced to 50 mmHg. Before and after induction of anaesthesia, 60 min after the beginning of the operation (end of SNP-infusion) and on the first postoperative day, IL-6 plasma concentrations were determined by ELISA. The percentages of B-, T-lymphocytes, T-helper, T-suppressor cells and HLA-DR positive (activated) T-lymphocytes were examined by direct immunofluorescence using monoclonal antibodies. On the first day after surgery IL-6 plasma concentrations were significantly elevated in the SNP-group compared to preoperative values. In this group the values were higher than in control patients [30.5 (10.9-47.5) pg/ml vs. 17.4 (8.5-21.5) pg/ml]. The percentage of HLA-DR positive T-cells was 25.8 +/- 4.9% in the patients with SNP on the first postoperative day; it was significantly higher than in control patients [16.5 +/- 3.7%]. We conclude that SNP-administration increases percentage of activated T-cells and IL-6 secretion.