A physicochemical and mathematical model of the mechanism of physiological action was assumed, and the model was applied on prostaglandins. A conformation-action relationship was studied using the model, and high values of correlation coefficients were obtained. Biological activities of prostaglandins on rat uterus, guinea pig ileum, and rabbit jejunum were explained using the model. Active sites of prostaglandins were derived from the mathematical model. They are the oxygen atom bonded to the carbon atom C₁, the oxygen atom bonded to C₉, the oxygen atom bonded to C₁₅, and the carbon atom C₅ or C₂₀.