Our previous work with lithium clearance method indicated that the diuresis and natriuresis induced by intracerebroventricular administration of hypertonic saline (icv. HS) resulted from an increase in glomerular filtration rate and a decrease in water and sodium reabsorption in the proximal tubules. In the present study, we further observed the effect of icv. HS on the glomerular filtration rate and proximal convoluted tubular and loop reabsorption in the superficial nephrons with micropuncture technique. After icv. HS, single nephron glomerular filtration rate increased from 39.6 +/- 1.9 nl/min to 48.8 +/- 2.0 nl/min (P less than 0.001), late proximal convoluted tubular fluid flow rate increased from 20.5 +/- 1.4 nl/min to 28.4 +/- 2.0 nl/min (P less than 0.01), and the ratio of tubular fluid inulin concentration to plasma inulin concentration decreased from 1.98 +/- 0.08 to 1.69 +/- 0.05 (P less than 0.01). Calculated fractional proximal convoluted tubular fluid reabsorption decreased from 49.2 +/- 2.2% to 41.7 +/- 1.8% (P less than 0.001), while calculated absolute proximal convoluted tubular fluid reabsorption did not change significantly. These results coincided with the observation with lithium clearance method. In addition, icv. HS increased absolute loop fluid reabsorption but decreased fractional loop fluid reabsorption. The present data suggest that stimulation of the brain osmoreceptor increases the glomerular filtration rate and decreases the reabsorption capacity of the proximal convoluted tubule in the superficial nephrons, both these responses result in an increase in the load of the loop of Henle with a secondary change in loop reabsorption. However, the results of the present study do not rule out the possibility that icv. HS may also inhibit the loop reabsorption directly.