In the course of cryopreservation cells are placed under extremely unfavorable external environment conditions [4]. This happens first at the stage of equilibration with the cryoprotective media, which contain, as their main component, cryoprotectors, which are diphilic compounds capable of modifying the structure and surface properties of biomembranes substantially as a result both of changes in the properties of the solution [5] and of direct binding with the membrane [5, 6]. In the case of nonpenetrating cryoprotectors (sucrose, polyvinylpyrrolidone, PEO-1500, etc,) to maintain osmotic equilibrium the cryoprotective media do not contain salts or contain them only in exceedingly small amounts. To judge from the available data [3, 8], this may lead to the development of a diffusion transmembrane potential (TMP), and even of electrical failure of the membranes. Subsequent freezing worsens the dielectric properties of the biomembranes, which facilitates the development of the above-mentioned process even more. In this investigation, to optimize conditions of cryopreservation, the possibility of development of a diffusion TMP in cryoprotective media prepared on the basis of nonpenetrating cryoprotectors, and the creation of conditions preventing this phenomenon were studied. EXPERIMENTAL METHOD Experiments were carried out on erythrocytes from fresh (stored 1-3 days) donors' blood. Sucrose and PEO-1500 were used as cryoprotectors. Erythrocytes were washed by centrifugation 3 times in physiological saline to remove blood plasma. The pH of the medium was carefully maintained at 7.4 during this procedure. The packed cells (0.2 ml) were transferred to a cuvette with continuously mixed medium containing i0 ml of a mixture of unbuffered solutions of 0.5 M sucrose or 0.13 M PEO-1500, pH 7.4, containing different quantities of NaCI. The kinetics of the change in pH of the medium was recorded by an ESL-41G-04 electrode on a KSP-4 automatic writer [3]. The dynamics of the outflow of intracellular K + into the external medium was investigated in these same blood samples by flame photometry, as described in [I]. The erythrocytes were rapidly frozen by immersing ampuls containg 1.5-2 ml of blood in liquid nitrogen. The rate of freezing under these circumstances was 200-400 ~ C/min. The cells were thawed on a water bath at 37~