Iron is examined for its ability to modify Ca2+ transport across sarcoplasmic reticulum (SR) and to alter the binding of [3H]ryanodine to its high-affinity site on the Ca2+ release channel complex of SR preparations from rat heart. Iron(III) (added as ferric chloride) has negligible activity on active Ca2+ accumulation into SR and on the binding of [3H]ryanodine. In contrast, Fe(II) (added as ferrous sulfate) is a potent inhibitor of both Ca(2+)-induced Ca2+ release (IC50 of 29 microM) and DXR-induced Ca2+ release (IC50 of 14 microM). Iron(II) enhances the rate of active Ca2+ uptake into SR vesicles, mimicking the actions of the known SR Ca2+ channel blocker ruthenium red. The underlying mechanism of Fe(II) on SR Ca2+ transport is shown to be a direct and potent action on the ryanodine receptor. Fe(II) inhibits the binding of [3H]ryanodine when assayed in the presence of 5 microM Ca2+ with an IC50 of 4 microM and in an apparently cooperative manner (nH = 1.7). In the presence of physiological (1 mM) Mg2+, Fe(II) decreases the sensitivity of ryanodine receptors toward activation by Ca2+ shifting EC50 from 18 to 35 microM in the absence and presence of 5 microM Fe(II), respectively, without significant decrease in maximum [3H]ryanodine occupancy. In the presence of 5 microM Ca2+ and 1 mM Mg2+, Fe(II) decreases the potency of doxorubicin (DXR) on [3H]ryanodine binding (shifts EC50 from 8 to 24 microM in the absence and presence of 5 microM Fe(II)). These results suggest that Fe(II) competes with Ca2+ at the activator sites on the channel complex. The actions of Fe(II) on ryanodine receptor function is not correlated with membrane lipid peroxidation of SR vesicles since Fe(II) does not produce detectable changes in malondialdehyde using the thiobarbituric acid assay. These results demonstrate a direct inhibition of the Ca2+ release channel of cardiac SR by Fe(II) which may be important in pathological states of the heart during iron overload.