1. We have investigated the effects of varying flow velocity (U) upon permeability to potassium ions (PK) of single perfused mesenteric venules in anaesthetised rats. PK was estimated using a development of the single bolus microperfusion technique at chosen flow velocities in the range of 300 to 6000 microm s-1. 2. In an initial study on 12 vessels, there was a strong positive correlation between PK and U. This was described by the relation: PK = 0.0053U + 8.86, where PK and U are both expressed in micrometres per second (microm s-1). 3. The addition of the nitric oxide (NO) synthase inhibitors (20 micromol l-1) N G-monomethyl-L-arginine (L-NMMA) and N G-nitro L-arginine (L-NNA) to the superfusate abolished the positive correlation between PK and U. The addition of D-NNA (20 micromol l-1) did not change the relation between PK and U where the median value for the slope of the relation was 57.7 (+/- 58.7 interquartile (IQR)) x 10-4 (n = 4). The addition of L-arginine (200 micromol l-1) restored the relation between PK and U where the slope of the relation was increased from 3.9 (+/- 16.3 IQR) x 10-4 to 69.2 (+/- 13.5 IQR) x 10-4 (n = 7). 4. The addition of the guanylate cyclase inhibitor LY83583 (10 micromol l-1) abolished the positive correlation between PK and U (n = 6). 5. Our data suggest that the flow modulates the potassium permeability through the walls of single perfused rat mesenteric venules via a NO-cGMP-dependent process.