1. Aprotinin, a potent kallikrein inhibitor, was given to conscious rats with and without expansion of the extracellular fluid volume with isotonic saline.
2. In noa-expanded rats aprotinin had no effect on arterial pressure, glomerular filtration rate (GFR), hippuran clearance, urinary flow rate, absolute sodium and potassium excretion or free-water clearance.
3. In volume-expanded rats aprotinin significantly reduced GFR, hippuran clearance, urine volume (V) UNaV, UKV and Cwater/GFR without effect on systemic arterial pressure.
4. Urinary immunoreactive prostaglandin E2 excretion significantly increased during the expansion phase but returned to below the control range during stable extracellular fluid volume expansion.
5. Aprotinin significantly suppressed urinary immunoreactive prostaglandin E2 excretion in non-expanded rats and in volume-expanded rats during the expansion phase, but not during stable expansion.
6. The results suggest that the kallikrein-kinin system may contribute to changes in renal function during extracellular volume expansion. This action may not necessarily be associated with changes in renal prostaglandin E2 activity.