A previous study demonstrated a strong but short-lasting suction through the catheter eyes by a hydro-dynamically generated negative pressure fluctuation terminating bladder evacuation in some frequently occurring circumstances of indwelling catheter drainage. This report regards the biological effect. Fifteen successive evacuations on such drainage conditions in each of 4 anesthetized pigs were followed by cystectomy and histological examination. All bladders presented small swollen areas, histologically showing localized mucosal elevations dominated by edema of lamina propria and submucosa, occasionally with urothelial thinning or defects. Drainage with suction prevented in 3 animals caused normal bladders. The changes were similar to those following hydro-statical suction and much like those of the "polypoid cystitis" so commonly occurring with indwelling catheters. This suggests both types of suction by ordinary drainage as a major pathogenetic factor in the latter condition. The clinical significance and the occurrence during regimes of straight drainage or intermittent clamping are discussed.