Isolated arteficially perfused rat kidneys prepared as described by Franke et al. (1971) were perfused for 60 min with solutions of Haemaccel, Dextran 40, Pluronic-F-108, or hydroxy-aethyl starch in a single pass system. The glomerular filtration rate (GFR) of the Haemaccel or Dextran 40 perfused organs amounted during the first 30 min to 0.58 ml X g-1 X min-1 and 0.47 ml X g-1 X min-1 respectively. Using Pluronic-F-108 or hydroxy-aethyl starch GFR rose to 0.94 ml X g-1 X min-1 and to 0.85 ml X G-1 X min-1. With Haemaccel or Dextran 40 solutions a mean tubular Na-reabsorption of 75.4 mumol X g-1 X min-1 and of 59 mumol X g-1 X min-1 respectively was determined. Employing Pluronic-F-108 or hydroxy-aethyl starch a mean sodium net transport of 92.6 mumol X g-1 X min-1 in both experimental groups was obtained. The differences described in the functional capabilities of Haemaccel or Dextran 40 and of Pluronic-F-108 or Hydroxyethyl starch perfused kidneys are in good accordance with morphological changes in the ultrastructure. The most striking morphological deviations were found in proximal tubules of those kidneys perfused with Haemaccel solutions. On the other hand after perfusion with hydroxyethyl starch only very few morphological alterations could be detected.