Investigations of fine changes of nephron elements in acute pyelonephritis against the background of urolithiasis in 19 patients and 43 rabbits showed the developing morphological lesions in the kidneys to be due to combined effect of the inflammatory and ischemic factors. The inflammation began with the development of serous oedema leading to destruction of tubular epithelium and impairment of urine passage. These lesions contributed to transformation of the serous inflammation into purulent one with formation of histolysis foci. The ischemic factor progressed with the advance of the inflammatory process and was aggravated by the development of foci of histolysis and hephrohydrosis and subsequently of vascular-parenchymatous block at the tubular level.