The effect of early bilateral pyelonephritis on urinary concentrating ability was studied in rats injected intravenously with enterococci or Staphylococcus aureus and in rats inoculated with Escherichia coli into the medullae of both kidneys. The mean maximum urinary osmolality of normal rats was 2352 mOsm/kg of water. Inoculation of E. coli caused reversible pyelonephritis with sterilization of the kidneys within 12 wk. By 1 day after injection the mean maximum urinary osmolality had decreased to about 1100 mOsm. remained at this level for 3 wk, and then rose to normal by 12 wk. After injection of enterococci and staphylococci, the mean maximum urine osmolality decreased over 3-4 days to about 1000 and 800 mOsm respectively. In the enterococcal infection (which is chronic) the maximum urine osmolality remained about 1200 mOsm for at least 12 wk whereas in the staphylococcal infection (which is reversible) the osmolality gradually rose.