The effect of pH and different concentrations of added monovalent and divalent cations on translation in poliovirus-infected HeLa cells has been examined. A strong effect on protein synthesis was observed when the concentration of sodium ions was modified. If cells were placed in a hypotonic medium after virus adsorption, no shut-off of cellular translation took place, nor were viral proteins synthesized. An increase in the multiplicity of infection partially overcame this effect. Reversal of the shut-off of cellular translation and inhibition of viral protein synthesis was achieved when cells were placed in hypotonic medium 2 h after infection. Modification of divalent cations (calcium or magnesium) had little or no effect on the pattern of translation. On the other hand, acidic pH (below 6) inhibited both cellular and viral protein synthesis, whereas basic pH had no influence. During infection the synthesis of poliovirus proteins reached a maximum at about 4 to 6 h and then declined. This inhibition of viral translation was partially prevented if cells were placed in a medium containing a high concentration of potassium although the cytopathic effect was prevented. These results indicate that viral protein synthesis and the cytopathic effect were, to a large extent, influenced by the external monovalent ion concentration.