The importance of tissue storage as a method of inactivation of circulating catechol amines has been assessed by measuring the amount of unchanged hormone remaining in the mouse 30 min after the injection of various doses of [(3)H]-adrenaline or [(3)H]-noradrenaline. The results show that this method of inactivation is quantitatively more important for noradrenaline than for adrenaline at all dose levels studied, and that for both hormones storage is relatively more important at physiological dose levels (3 to 30 mug/kg) than at higher dose levels (150 to 300 mug/kg). The results obtained after the simultaneous injection of various doses of [(3)H]-adrenaline and [(14)C]-noradrenaline show that under certain conditions the two hormones compete for entry into the tissue storage sites. The possible nature of the mechanisms by which circulating catechol amines enter the tissue stores is discussed in the light of previous findings on the uptake of catechol amines by tissues in vitro.