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Abstract— Quantitative studies on the interactions of adenosine‐triphosphate and several biogenic amines with magnesium ion have been carried out in an attempt to correlate the thermodynamic stabilities of the metal‐binding of the amines with the in vivo affinities of the amines for granule‐binding. Equilibrium data indicate that in each of the ternary chelate systems (viz. Mg2+‐ATP‐amine), the predominant reaction in the pH range 3.0–7.0 is the formation of a magnesium‐ATP chelate with a stability constant, log KML=3.22 ± 0.02. Each of the biogenic amines coordinates with Mg2+‐ATP system in the pH range 7.0–10.5 to form the mixed ligand chelate (or ternary chelate), Mg2+‐ATP‐amine(1:1:1). The stability constants for the binding of the amines with Mg2+‐ATP are: (i) norepinephrine (NE) = 2.34 ± 0.32; (ii) epinephrine (E) = 2.95 ± 0.08; (iii) dopamine (DA) = 3.05 ± 0.06; (iv) octopamine (OA) = 1.93 ± 0.12; (v) 6‐hydroxydopamine (6‐HDA) = 2.42 ± 0.14; (vi) 3‐methoxynorephedrine (MeN) =2.76 ± 0.09; (vii) amphetamine (AA) =2.09 ± 0.05; (viii) tyramine (TA) = 2.60 ± 0.04; (ix) phenylethylamine (PEA) = 0. A general correlation is indicated between the stability constants (binding strengths) of the amine chelates and the metal‐binding functionalities of the amines on the one hand and their vesicular binding characteristics in in vivo systems on the other (Carlsson and Waldeck, 1966). The Mg2+‐ATP‐dependant amine storage mechanism of KIRSHNER (1962a;b) and Carlsson, Hillårp and Waldeck (1963) is discussed both in the light of the data on metal chelate stability and of a significant modification of metal coordination hypothesis.