OBJECTIVE To determine the influence of distension and intravesical KCl on vesical blood flow in the normal human bladder. Subjects and methods Nine normal volunteers underwent comparative cystometry (NaCl vs 0.2 mol/L KCl; filling rate 50 mL/min). Peak systolic blood flow velocity (PSBFV) and end-diastolic blood flow velocity (EDBFV) were measured in several intramural arteries at a filling volume of 50 mL and at maximum cystometric capacity (Cmax). For these measurements a colour Doppler unit fitted with an endorectal probe was used. The resistance index (RI) was defined as (PSBFV-EDBFV)/PSBFV. RESULTS In the presence of NaCl, the mean PSBFV increased significantly from 9 cm/s at 50 mL to 17 cm/s at Cmax (512 mL). Compared with NaCl, KCl induced a significantly higher mean PSBFV at 50 mL (15 cm/s). With increasing distension the rise in PSBFV with KCl filling (Cmax 478 mL) was nearly parallel to that obtained on NaCl filling (mean 22 cm/s); the RI did not change. CONCLUSIONS Bladder distension and intravesical KCl significantly increased the PSBFV. The unchanged RI indicated a concomitant increase in perfusion rates. Hence, the composition of urine (high potassium concentrations and hyperosmolarity) and its storage govern the autoregulation (independent of cardiac output) of vesical circulation, probably by reflexive pathways. These findings provide further evidence for prevesical arteriovenous shunts.