Experimental, ascending acute pyelonephritis in rats was produced by injecting 0 x 5 ml of 10(9) bacteria/ml into the urinary bladder via the urethra. No traumatic manipulation of the ureters of kidneys was necessary. A grading system for kidney lesions based on macro- and microscopical examination was used. The capacity of different Escherichia coli and proteus strains to induce acute pyelonephritis was tested, and the E. coli 06K13H1 strain and the Proteus mirabilis 03H1 strain were especially capable of causing urinary tract infection. For the P. mirabilis 03H1 strain, a dominance of right kidney lesions was noted in contrast to the E. coli 06K13H1 strain which did not show any side preference.