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To test our hypothesis that a circulating factor(s) may be causing the renal salt and urate wasting in patients (pts) with intracranial diseases, we exposed rats to the plasma of these patients and studied sodium and lithium transport. We selected 21 neurosurgical pts, 13 of whom had increased fractional excretion (FE) of urate, and 14 age and sex-matched controls. Plasma from pts and controls were injected IP (0.5 mL) and infused, 0.2 ml prime and 1.8 mL at 0.01 mL/min, to Sprague Dawley rats anesthetized with Inactin. Renal transport of sodium (Na), lithium (Li) and potassium (K) was determined. There were higher mean +/- SEM for FENa, 0.59 +/- 0.07% vs 0.29 +/- 0.05%, P < 0.01, FELi, 36.6 +/- 1.9% vs 24.0 +/- 1.6%, P < 0.001 and K excretion rates, 1.69 +/- 0.13 vs 1.31 +/- 0.09 mumol/min, p < 0.02, in rats infused with plasma of pts as compared to controls, respectively. FENa decreased with increasing dilution of plasma of 2 pts with ICD. There was no difference in mean weight of rats, blood pressure, urine flow rate or insulin clearance between pts and controls. These data suggest that pts with ICD have a plasma factor(s) which decreases net Na, Li and K reabsorption.