The present investigation is the continuation of our prior clinical studies on the content of parathyroid hormone (PTH), its paracrine analog, parathyroid hormone-related protein (PTHrP) and electrolytes in blood of patients with heart failure. The results of these studies formed the basis for the nomination of the hypothesis on PTH and PTHrP compensatorymodulating effect on the contractile activity of heart. The objective of this study is to elucidate the mechanism of the compensatory-modulating effect of PTH on heart functional activity, which is realized by the study of effective doses of PTH by pharmacological analysis, using different inhibitors. The dose-dependent effect of PTH on the heart contraction rate and amplitude is studied on the frog isolated heart by the method of non-invasive registration of heart contractile activity. The method is based on the photoelectric principle of the reflected from the contractile object light ray transformation into an electric signaling. It is shown that the most effective dose that has positive chronotropic and inotropic effects on heart is 10 -10 M hormone. To clarify the mechanism of PTH physiological dose action on the contractile activity of heart PTH 1-34 is combined with Ca-channel as well as phosphodiesterase blockers. The mentioned substances are applied based on the fact that PTH effect on target cells is mediated by secondary messengers, particularly calcium ions and cAMP. Based on the data obtained by combination of hormone with Verapamil (10 -5 M) and Theophylline (10 -4 M), we concluded on the involvement of calcium ions in the realization of chronotropic and cAMP in the inotropic effects on the heart.