SUMMARYPreviously we have presented evidence for a direct relationship between post-mitotic new membrane formation and changes in the electrical membrane characteristics during cytokinesis of Xenopus eggs [l, 21. In the present study the phenomena underlying the hyperpolarization of the electrical membrane potential during cytokinesis were investigated. Total Na+ and K+ contents at the onset of the first and second cleavage were measured independently by flame spectophotometry and by means of ion-selective electrodes. Total Cl-content was measured by the latter method only. The water content was determined from the difference between wet weight and dry weight. 3H,0-influx experiments yielded an independent estimate of the water content (0.737 ,ul/egg), a rate constant for the influx of 1.412 x 10e3 set,-r and a low water permeability of 1.87 x 1O-5 cm set-I. They furthermore revealed the absence of intracellular water compartmentation. The Naf, K+ and Cl-concentrations remained constant during first cleavage at 58.6, 87.3 and 62.6 mM/l cell water, respectively. The membrane potential (E,), the membrane resistance (R,), and the intracellular ion activities of Na+, K+ and Cl-(a;,, ak and a$ were measured simultaneously and continuously during cleavage, using conventional glass microelectrodes and ion-selective microelectrodes. a' na showed an increase from 19.4 to 22.4 mM concomitant with the hyperpolarization of Em and the decline of R,. a; and a& remained constant at 51.4 and 53.1 mM, respectively. From the calculated activity coefficients it was concluded that all Cl-ions were free, whereas 30 % of the Kf ions and 60 % of the Naf ions were bound. The influence of changes in the ionic composition of the medium on E,,, was analysed in the uncleaved egg, in normally cleaving eggs, and in eggs cleaving outside the vitelline membrane. The latter conditions leads to exposure of the whole area of newly formed membrane to the medium. The cell membrane of the uncleaved egg exhibits no permselectivity. In normally cleaving eggs the relative permeability P&Px was 0.73, while in eggs cleaving outside the vitelline membrane it was 0.19. It was concluded that the changes of E,,, during cytokinesis are due to the insertion of a part of the newly formed membrane into the cell surface. The K+ permeability of this new membrane is at least five times greater that that of the pre-existing membrane. The possible role of the hyperpolarization of E,,, in the regulation of the cell cycle is briefly discussed.In previous reports [2, 31 the characteristic changes of the electrical membrane potential (E,) and the electrical membrane resistance (R,) during the first cytokinetic cycle of Xenopus laevis eggs were described. A temporal and causal relationship was found between these changes and new membrane formation. Ihe electrical membrane characteristics start to change at 6-8 min from the onset of the cleavage. From that time on morphological signs of the incorporation of new membrane material in the bottom of the cleavage furrow are ...