Abstract Male Sprague-Dawley rats carrying indwelling jugular catheters were used to compare two methods of blood collection, catheterization or decapitation, for obtaining the plasma levels of several electrolytes. Decapitation resulted in higher levels of Ca 2+, Mg 2+, and K + (18%, 23%, and 35% respectively) compared to catheter values. In a second experiment, catheterized rats were used to examine the effect of immobilization and footshock on plasma electrolytes. Both forms of stress elevated Mg 2+, whereas only immobilization elevated Ca 2+. Post-stress K + levels were significantly different than control values. These experiments demonstrate that catheterization must be used to obtain true plasma electrolyte levels, and that electrolytes may be selectively altered depending on the nature of the stressor.