Plasma, blood, and urine volumes, renal kallikrein, and arterial pressure were measured in control and renal hypertensive rats in order to study the role of the renal kallikrein system in regulating arterial pressure and its relation with the alterations in water handling observed in hypertension. A decrease in kallikrein content of the kidney (157 +/- 17 versus 236 +/- 16 ng bradykinin equivalents per gram of tissue in control rats) was associated with an increase in plasma volume (38.0 "/- 1.6 versus 32.0 +/- 0.9 ml/kg body weight in control rats) and an increase in urine volume (45.5 +/- 4.9 versus 20.3 +/- 1.6 ml/kg body weight per 24 hours in control rats). No linear correlation was found between these factors and the arterial pressure of hypertensive animals. These findings support the hypothesis that changes in renal kallikrein are more directly related to water and electrolyte metabolism than to the arterial pressure regulation. Our results also suggest an interaction between the kallikrein-kinin and the renin-angiotensin-aldosterone systems. The possible relations of both enzymatic systems to the regulation of arterial pressure and of water-electrolyte handling are summarized schematically.