«Wet belly» disease of fur animals, the sables particularly, has little been studied, and the data have been mostly limited to publications in 1960-1970. In our examinations carried out earlier on the sables with urinary incontinence, the incidence of anemia, anorexia, the inflammation in gastrointestinal tract, and the weight loss were also found, indicating the urinary incontinence not to be the only symptom of the disease. This article gives new information for histological changes in the urinary bladder of 7-month black sables, injured by «wet belly» (a «syndrome of urinary incontinence»). The catharal and haemorrhagical cystitis were indicated as possible etiological factors of the disease. Post-mortem examination of 76 sables with «wet belly», or «syndrome of urinary incontinence», showed that 28 % of the males, and 41 % of the females have various types of inflammation in the urinary bladder. Using morphometric analysis, a loss of the urinary bladder mucosa and muscle layer was demonstrated in the urinary bladder of sick animals if compared to healthy sables. Authors suggest that the phenomenon is probably related to high epithelium exfoliation and bladder dysfunction, such as atony and dilatation in the muscle layer of urinary bladder. In the young sables, injured by haemorrhagic cystitis with the symptoms of «wet belly» disease, an increase of glycosaminoglycans in the urinary bladder tissue was detected by hystochemical methods, indicating activation of local defense mechanisms due to the inflammation. These facts support a hypothesis, whereby a various form of cystitis may account for the development of «wet belly» disease in young sables.