In 2 recent studies the authors compared the protein synthesizing capacities of mengovirus infected Ehrlich ascites tumor (EAT) cells with those of their corresponding postnuclear supernatants as a function of time postinfection (Egberts et al., submitted for publication). This report describes the investigation of the intracellular energetic and ionic conditions in mengovirus infected EAT cells and discuss their relevance to the regulation of protein synthesis in the middle and late phases of mengovirus infection. Mengovirus infection of EAT cells caused a change of the intracellular ATP concentration. It increased by 35% within the first 3 hr postinfection and then declined to zero within the next 5 hr. The decrease in the ATP concentration was due, at least in part, to leakage of ATP into the medium, where it could be demonstrated by the luciferin luciferase assay. Gross leakage of ATP was observed at 4.5 hr postinfection, concomitant with the production of the first intracellular, infectious virus particles. A similar concentration decrease was detected for Mg2+, the polyamines, and K+, whereas an increase in the Na+ concentration was observed. The intracellular Mg2+ concentration varied synchronously with the ATP level, rising by 16% during the first 3 hr postinfection and then progressively falling to lower values in the late period of the infectious cycle. After an initial slight enhancement, the putrescine, spermidine, and spermine concentrations declined at about 1.5 hr postinfection. Whereas the intracellular K+ concentration increased by 17% during the first hr postinfection, the Na+ concentration diminished by the same value within the same time period, leaving the internal ionic strength unchanged early in infection. Three hr after the beginning of virus infection, there was a rapid decline of K+ and enhancement of Na+ within the cell. These alterations of the intracellular energetic and ionic conditions seem to be, at least in part, responsible for the cessation of virus specific protein synthesis in mengovirus infected EAT cells commencing 3 to 3.5 hr postinfection.