Epinephrine injected rapidly intravenously disappears from the plasma of anesthetized dogs in 4 to 6 minutes. Mean half-time for this disappearance rate is 23 seconds. Epinephrine administered by a continuous intravenous infusion approaches constant plasma levels in 30 min. when given at a rate of 6 ug/kg/min. Following cessation of a constant infusion the plasma epinephrine concentration falls rapidly for 4 or 5 min., then slowly during the subsequent hour. The half-time for the rapid fall is 38 sec. and for the slow phase is 23 min. Inconsequential amounts of epinephrine are excreted in the urine during this period of time.