The effect of delta-sleep peptide (DSP) deficiency on the parasympathetic regulation of the heart rate was studied on 35 rabbits. It was established that the injection of an-serum (titer-1:2000-1:3000) leads to the attenuation of parasympathetic influences: heart rate increase in freely behaving animals and a decrease in negative chronotropic effect with direct vagus irritation. Antiserum, like DSP, administration causes practically no damage of the myocardial ultrastructure.