The purpose of this study was to determine the influence acute stress on the plasma levels of active and inactive renin in the rat. Active and inactive plasma renins were measured in calm unanesthetized normal Wistar rats (indwelling catheter) and compared to values obtained after handling, anesthesia and decapitation. All maneuvers resulted in an increase of active renin. Only anesthesia significantly reduced inactive renin. It is concluded that acute stress can significantly change the ratio of active and inactive renins in rat plasma. These changes must be taken into consideration when the influence of "physiological or pharmacological" alterations of these enzymes are under investigation.