The minutes immediately after cessation of strenuous exercise have been termed the “vulnerable period” regarding the occurrence of lethal cardiac arrhythmias. In the present study, we analyzed the rates of change with a high degree of temporal resolution of factors known to be associated with induction of arrhythmic activity. Thirteen subjects walked and ran on a treadmill to the point of exhaustion after a modified Bruce protocol. Samples were drawn from the antecubital vein immediately before exercise, at the moment exercise stopped, and at 1, 2, 3, 5, and 10 minutes of recovery. Plasma potassium concentration rose during exercise from 3.98 ± .05 to 5.09 ± .09 mEq/L. After the exercise period, plasma potassium fell rapidly. The mean maximum rate of change was −0.54 ± .04 mEq/L/min, and −0.88 ± .08 mEq/L/2 min. Plasma epinephrine concentration increased from 63 ± 14 to 497 ± 41 pg/ml and norepinephrine increased from 503 ± 57 to 2800 ± 519 pg/ml during exercise. During the first minute after cessation of exercise, the concentrations of both catecholamines increased still further. We propose that the rapid decline of potassium concentration while catecholamine levels are elevated to near maximal levels may contribute to the vulnerability of some individuals to post exercise arrhythmias.