Despite the wide use of laparotomy to study kidney function, the possible influence of this procedure on systemic and renal parameters in septic rats is unknown. We studied this in anesthetized Wistar rats with and without endotoxin shock (1 h Escherichia coli O 127.B8: 8 mg.kg-1 infusion). We also compared clearance of creatinine and inulin to measure glomerular filtration rate (GFR). Laparotomy attenuated the endotoxin-induced decrease in cardiac output and abolished the increase in systemic and renal vascular resistance, while renal plasma flow was maintained. Better perfusion in other organs as well was indicated by a more gradual increase in arterial lactate concentration and less intestinal damage. By contrast, GFR decreased considerably during endotoxemia, irrespective of laparotomy. This change in GFR could be reliably assessed using creatinine clearance. The ratio of creatinine-to-inulin clearance averaged between .5 and .75. Renal ATP content did not change and the endotoxin-induced increase in the number of granulocytes lodged in glomeruli was not affected by laparotomy. In conclusion, our study indicates that laparotomy significantly influences the vascular effects caused by endotoxin. Laparotomy also revealed an effect of endotoxin on GFR, independent of renal blood flow.