“…This suggests that 70 to 75% of the endogenous noradrenaline in the rat heart is present in a third intracellular pool which exchanges at a negligible rate with exogenous noradrenaline or with either of the other two intracellular pools during the limited duration of these experiments. These findings are consistent with the growing evidence that the noradrenaline store in sympathetically innervated tissues cannot be considered to be a single homogenous entity (Trendelenburg, 1961(Trendelenburg, , 1963Campos & Shideman, 1962 ;Kopin et al, 1962;Potter, Axelrod & Kopin, 1962;Iversen & Whitby, 1963), but it will require more experiments to delineate the characteristics of the various intracellular pools more precisely. I am grateful to the Medical Research Council for a scholarship and for a grant for animals and materials, to Dr E. Muscholl, University of Mainz, and to Dr J. Raventos, of I.C.I., for their generous gifts of (+)-noradrenaline, and to Professors L. G. Whitby and A. S. V. Burgen for much helpful advice and criticism.…”