The effect of the Krebs solution tonicity on the electrical and mechanical activity of smooth muscles has been studied while adding 1-5% volume of mineral water Naftusya, its artificial salt analog, and distilled water into the above solution. It is shown that effects observed are induced by the medium tonicity changes rather than by biologically active components of the mineral water. The smooth muscle cells of the rat portal vein are sensitive osmometers which affect hypotonic Krebs solution by 1%.