Bioglycans isolated from chaga in a concentration of 0.0001% reduced frequency of action potential in venous sinus cells of frog heart during the first 15–30 min of exposure, then this parameter increased by 10% per hour over 3.5 h, and was 41±3 min⁻¹ from the 4th to the 20th hour of incubation. The frequency of action potentials in heart strips in the chaga extract was 40% higher than in Ringer's solution. The effect of chaga bioglycan is probably associated with adsorption on myocyte membranes. Binding of Ca²⁺ to bioglycans observed during the first 30 min inhibited efflux of intracellular Ca²⁺.