Abstract Ca 2+ accumulation at pH 6.8 by isolated rabbit heart microsomes derived chiefly from sarcoplasmic reticulum was investigated by a quench-flow technique. The reaction was terminated at preset times by addition to the reaction mixture of an equal volume of 10 to 50 mM ethyleneglycol-bis-( β-aminoethyl ether)- N, N′-tetraacetic acid buffered at pH 6.0. The initial velocity of Ca 2+ accumulation by microsomal preparations exhibiting a steady state Ca 2+ accumulation of 25.6 nmol Ca 2+/mg increased from 3.67 to 33.4 nmol Ca 2+/mg · s as the free Ca 2+ concentration was raised from 0.2 to 18.9 μM. Preincubation of the cardiac microsomes with a partly purified soluble cardiac cyclic AMP-dependent protein kinase, MgATP, and cyclic AMP lead to a significant increase in the initial Ca 2+ accumulation rate. The amounts of Ca 2+ that were found to accumulate in the first 200 ms of the reaction are comparable to the quantities of the ion that according to literature data need to be removed from the myofilaments and the myoplasm for induction of relaxation of the myocardial fibers.