Three explanations are given in human medicine for the origin of this lesion. Some observers assume a direct transmission of bacteria up the ureter from an infected bladder or urethra, in a direction contrary to the urinary current. This, it is believed by some can occur only in the presence of physical obstruction, stone, kink, or pressure of adjacent masses upon the ureter, whereby its blood supply is damaged and infection facilitated. Others would explain the path of infection as the lymphatics of the ureteral wall which are infected at the opening in the bladder by a deep seated cystitis or by infection from a periproctitis or from the female genitalia. Still another explanation is offered by those who do not credit ascending infections. They would have it that pyelitis arises from bacteria in the blood stream and only in the presence of injury (_calculus_), ureteral dilation (slowly progressive stenosis, or kinks, floating kidney, pressure by pregnant uterus) and similarly operative factors. There should be excluded from this category cases of chronic nephrolithiasis, and of stone only in the pelvis. Under such conditions it is inevitable that a low grade of fibrosis with damage to the secretory structures should exist, even in the absence of active bacterial invasion. The cases are only important for our present subject when active bacterial infection is implanted upon them. Among our autopsies there have been found fourteen cases of ascending nephritis and seven cases of pyelitis; the most instructive examples are mentioned briefly as follows: Five of this twenty-one were associated with general infectious diseases (three septicemias) and presented no evidence of ascending obstruction. Two of this five were a bear and a fox, the former suffering with streptothricosis septicemia, the latter with distemper; one was a fox whose pelvic and renal lesion seemed entirely primary for no apparent focus was detected; two were birds with acute general infection. These cases seem therefore to be instances of primary pyelitis. The following group includes cases with inflammation low in the urinary tract. A fox had a cystitis, urethritis and colitis, a distinct edema and congestion being found in the pelvis around the rectum. Two opossums had cystitis, one due to a traumatic urethritis, the other secondary to a prostatitis of undetermined causation. A raccoon had a chronic cystitis with swelling and edema of the first parts of ureter. A parrakeet showed ureteritis, pyelitis and nephritis from simple cloacitis.