The pressures in the renal interstitial space seem to have important influence on the setting of the sensitivity of the tubuloglomerular feedback that controls the glomerular filtration rate (GFR), and on the rate of proximal tubular fluid reabsorption. Measurements were made of interstitial pressure conditions, GFR, renal plasma flow (RPF), urinary excretion of sodium and potassium, and plasma renin activities in dehydrated animals and normopenic controls, before and after saline volume expansion (5% of body weight and hour). Colloid osmotic pressure, estimated from the protein concentration in renal hilar lymph, was 7.5 mmHg in the dehydrated animals (controls 2.8 mmHg) and decreased to 3.1 (controls 1.7 mmHg) after volume expansion. The lymph flow rate was increased in both groups of animals after volume expansion. Interstitial hydrostatic pressure, measured in the subcapsular space, was 2–3 mmHg in dehydrated and control animals and increased to 3–4 mmHg after volume expansion. In dehydrated rats GFR and RPF was reduced to 60% of the control values, but after volume expansion they regained control values. After volume expansion, urinary excretion of fluid and electrolytes increased more in controls than in dehydrated rats. Plasma renin activity was dereased in both groups of rats after volume expansion. Thus, in dehydrated animals there was a high colloid osmotic pressure and a low hydrostatic pressure in the renal interstitium, while after volume expansion the oncotic pressure fell and the hydrostatic pressure rose. The effect of volume expansion was found to be dependent on the preceding volume balance situation in the ani