In anaesthetized 300 g rats, the influence of sympathetic nerve activity on the renal hemodynamics, glomerular filtration and fluid reabsorption was studied with direct stimulation at frequencies of 2 Hz and 5 Hz. The single nephron plasma flow at control conditions was 164 nl/min decreasing to 138 nl/min during 2 Hz and 68 nl/min during 5 Hz, reaching complete glomerular ischemia at about 10 Hz. At 2 Hz, the pressure drop over the two arterioles remained essentially unchanged, indicating an equal response to sympathetic discharge. At higher frequencies the afferent tone showed a more marked increase. The glomerular ultrafiltration decreased in parallel to the blood flow. The filtration fraction remained thereby constant at about 0.33. The fractional proximal fluid reabsorption up to the puncture site in early distal tubules showed a clear increase; the Tf/P-Inulin increasing from 6.0 to 7.1 and 7.2 for 2 Hz and 5 Hz, respectively. The absolute reabsorption decreased, however, and indeed not far from the decrement in glomerular filtration. It is concluded that sympathetic nerve activity acts in the direction of fluid conservation, by reducing the glomerular filtration and increasing the fractional reabsorption. The hemodynamic effects will play the dominant role even at 2 Hz stimulation.