Ruthenium red inhibits mitochondrial Ca2+ uptake and is widely used as an inhibitor of ryanodine-sensitive Ca2+ channels that function to release Ca2+ from the sarcoplasmic reticulum (SR) of muscle cells. It also has effects on other Ca2+ channels and ion transporters. To study the effects of ruthenium red on Ca2+ transport into the SR of cardiac muscle cells, fluorescence measurements of Ca2+ uptake into cardiac SR vesicles were made. Ruthenium red significantly decreased the Ca2+ sensitivity of SR uptake in a dose-dependent manner at concentrations ranging from 5 microM to 20 microM. There were no significant effects of ruthenium red on the maximum velocity or the Hill coefficient of SR Ca2+ uptake.