1. In normally hydrated rats prostaglandin F2α (PGF2α) in doses of 5 μg/100 g body weight given subcutaneously every 2 h (three times) induced a significant increase in urinary kallikrein activity, and in sodium, potassium and water excretion for 8 h after the first injection. In moderately hyperhydrated rats loaded 2·5% of body wt. with 0·5% NaCl solution, PGF2α produced similar changes in kallikrein activity and electrolyte excretion.
2. In normally hydrated rats prostaglandin E2 (PGE2) in the same conditions and doses as in 1 had no effect on kallikrein activity, showing a tendency to decrease potassium and water excretion.
3. PGE2 in doses of 5, 12·5 and 25 μg/100 g body wt. in overhydrated rats given 2·5% and 0·5% NaCl and 5% of tap water/100 g body wt. 1 h later, significantly increased kallikrein activity in the urine collected for 120 min after the injections. A significant decrease in potassium and water excretion was observed with the highest dose.
4. PGF2α, had no effect on kallikrein activity in overhydrated rats, but an increase in sodium and a decrease in potassium excretion was seen at the highest dose.
5. The different actions of PGE2 and PGF2α may be part of a regulatory mechanism associated with the kallikrein—kinin system which contributes maintainance of extracellular fluid homeostasis.