A critical evaluation of the functional properties of the isolated perfused rat kidney is necessary to assess the usefulness of the preparation for renal function studies. Clearance and micropuncture experiments in isolated perfused rat kidneys perfused with a plasmalike medium containing 7.5 g/100 ml albumin, glucose, and amino acids show that proximal convoluted tubule functions are well preserved. Proximal convoluted tubule reabsorption of organic substances, electrolytes, and fluid is near normal, the latter being directly related to the peritubular oncotic pressure. Superficial single nephron glomular filtration rate and glomerular permselectivity are also preserved. However, abnormalities in renal hemodynamics, urinary concentration-dilution, and excretion of fluid and electrolytes persist even in the best preparations. High renal perfusate flow, due mainly to the low viscosity of the perfusate, and altered distal nephron functions explain at least in part these abnormalities. Therefore, the isolated perfused rat kidney is a useful preparation to particularly study glomerular and proximal convoluted tubule functions. Recent development of a nonfiltering isolated perfused rat kidney model, with preserved renal perfusate flow and cellular integrity, also permits the study of transport and metabolic functions of proximal tubular cells independently of luminal events.