Abstract: The effects of cyproheptadine on Ca²⁺, Mg²⁺--ATPase activities and ⁴⁵Ca²⁺ uptake function in isolated canine myocardial sarcoplasmic reticulum were studied. Cyproheptadine at concentrations of 8×10^-6 mol/L～2×10^-4 mol/L showed hardly any influence on Ca²⁺ , Mg²⁺-- ATPase activities of the myocardial sarcoplasmic reticulum in canine, but inhibited the enzyme activities (rate of inhibition: 39.85%, P<0. 01) at the concentration of 10^-3 mol/L. The myocardial sarcoplasmic reticulum was shown to take up ⁴⁵Ca²⁺ significantly in a time-dependent manner and the ₄₅Ca²⁺ uptake was 312. 79±22.25 nmol/mg protein at 30 min. Cyproheptadine was found to inhibit the Ca²⁺ uptake of sarcoplasmic reticulum with an IC₅₀ of 1.94×10^-4 mol/L.