A drastic modification in permeability to monovalent ions occurred in HeLa cells infected with poliovirus, starting in the period from the third to the fourth h post-infection. The bulk of poliovirus protein synthesis took place from the third to the sixth h in cells in which the concentration of monovalent ions in the cytoplasm had changed considerably compared to uninfected cells. Under our conditions of infection (HeLa cells grown in monolayer), poliovirus translation continued beyond the eighth h. Modification of permeability to 86Rb+ ions induced by poliovirus infection was prevented by the addition of 2 mM-guanidine at the time of infection. However, when poliovirus replication was allowed to take place for 1 h before addition of guanidine, the membrane did become modified to some extent. The degree of leakiness to 86Rb+ ions increased when guanidine was added later. The blockage of membrane leakiness by early addition of guanidine was overcome by the addition of choline. The inhibition of cellular protein synthesis, which occurred early in infection or in the presence of guanidine, did not coincide with the modification of permeability to 86Rb+ ions. Viral protein synthesis was necessary in order to modify the membrane late in infection, since addition of 10(-5) M-cycloheximide during the first 2 h of infection prevented the leakiness to 86Rb+ ions observed from the fourth h after infection. Membrane potential, as measured by the lipophilic cation TPP+ (tetraphenylphosphonium), dropped from the fourth h post-infection. This change in the membrane was also prevented when viral gene expression was inhibited by the presence of guanidine.