The technique for X-ray microanalysis of frozen-hydrated bulk specimens was used to determine the intracellular and luminal fluid electrolyte concentrations in the proximal tubules of kidneys from chickens infected with infectious bronchitis virus. Eight days post-infection with this virus there were significant changes in the electrolyte composition when compared with values from normal control chickens. The intracellular sodium decreased from 43 to 36 mmol/l, the chloride fell from 41 to 31 mmol/l and the potassium went from 125 to 115 mmol/l. Sodium counts in the luminal fluid rose from .73 to 1.03 cps. These disturbances in electrolyte composition are consistent with alterations in sodium reabsorption in the proximal tubule due to decreased transport of sodium into the cells across the microvillus membrane. It appears that the Na-K-ATPase pump is unaffected. The results demonstrate the value of X-ray microanalysis methods for the study of electrolyte transport in pathologically affected cells and provide further information for the definition of viral-host cell interactions in the pathogenesis of viral disease. As a check on methodology two normal rat kidneys were analysed in the same way. Intracellular sodium and potassium concentrations were 22 and 138 mmol/l respectively.