One of most frequently used anesthetic agents is barbiturate derivatives. Pentobarbital or thiopental sodium have been used most frequently in the laboratory or clinical practice. There have been reports on the renal effects of barbiturate anesthesia in human and laboartory animals. Renal effects of thiopental sodium anesthesia, however, are still controversial. One of the discrepancies may be derived from the doses used. It has been reported that subanesthetic small dose of thiopental sodium influences the renal function directly. To clarify possible central effects of very small amounts of thiopental sodium on the renal function, experiments have been done in conscious rabbits. Thiopental sodium was infused into the lateral cerebroventricle for 10 minutes. Intracerebroventricular thiopental sodium induced increased urinary volume, glomerular filtration rate and renal plasma flow by doses of 0.1 ~ 1.0 mg/10 min/rabbit. Filtration fractions were not changed. Sodium, chloride and potassium excretions were increased by 0.065 mg/10 min/rabbit of thiopental sodium without significant changes of renal hemodynamics. Higher doses of thiopental sodium (0.1 ~ 1.0 mg/10 min/rabbit) induced greater increases of electrolytes excretion and renal hemodynamics. Free water clearance was not changed by thiopental sodium, but the fractional excretion of free water showed a tendency of decrease. Fractional excretion of sodium was increased by doses of 0.065 to 1.0 mg of thiopental sodium . Highly significant correlation between the changes of glomerular filtration rate and the changes of sodium excretion were found in the higher doses. Plasma renin concentration (activity) was not changed by the centrally administered thiopental sodium. Intravenous thiopental sodium, 1.0 mg/rabbit, induced no changes of renal function in conscious rabbit. These data suggest that intracerebroyentricular thiopental sodium can increase urinary sodium excretion directly by inhibition of sodium reabsorption in the renal tubules and/or indirectly by increasing the renal hemodynamics.