Abstract: AIM: To explore the molecular mechanism of the calcium sensitizing effect of oxyphenamone(Oxy). METHODS: Cardiac troponin C(cTn C) was purified from cattle cardiac muscle and labeled with fluorescent probe, dansyl chloride. The effect of Oxy on the affinity of cTn C to Ca²⁺ was studied and compared with those of MCI 154 and sulmazole(Sul). RESULTS: Oxy was shown to improve the affinity of Ca²⁺ dose dependently. Oxy 10 μmol·L ^-1 slightly shifted the cTn C—Ca 2+ affinity curve to the left, but showed no significance while Oxy 25 μmol·L^-1 significantly shifted the curve to the left. Oxy 50 μmol·L^-1 showed more potent action, shifting the affinity curve to the left by 0.2 pCa unit. In contrast, MCI 154 or Sul 50 μmol·L^-1 showed no significant influence on the affinity curve. CONCLUSION: These results suggest that the positive inotropic action of Oxy on cardiac muscle is, at least, partially mediated by increasing the sensitivity of cTn C to Ca²⁺, the calcium sensitizing effect of MCI-154 and Sul may be mediated otherwise besides directly increasing the affinity of cTn C to Ca²⁺.