It is a common observation that resistance sized arteries have the capacity to constrict in response to increases in intraluminal pressure; the myogenic response. The mechanisms that underlie myogenicity are not clearly defined although smooth muscle depolarisation and sensitivity to dihydropyridine antagonists appear strongly correlated with the myogenic response. Large arteries often lack the response, leading to the hypothesis that myogenic reactivity is inversely proportional to arterial diameter. Evidence exists to support such a relationship, for example, the arterial network of the hamster cheek pouch (Davis 1993) and rat cerebral arteries (Brekke et al 2002). In this study we tested the hypothesis that resting potentials recorded from rat mesenteric arteries at the sub-myogenic pressure of 20mmHgl correlate with arterial diameter; that smaller diameter correlates with depolarised resting membrane potential. Arteries were mounted in a pressure myograph bathed in PSS solution, bubbled with 95%O₂:5%CO₂, pH 7.4 at 37^oC. Diameter was monitored using video microscopy and edge-detection software (Living Systems Inc). Vessels were initially pressurised to the myogenic pressure of 80mmHg, 37°C for 1 hour to record any development of tone. After which the pressure was maintained at the sub-myogenic pressure of 20mmHg and the arterial myocyte membrane potential was measured, using sharp microelectrodes (130-180MΩ), filled with 3M KCl. Quoted diameters are passive ± sem (n animals); being measured in the nominal absence of calcium, at 80mmHg. The mean resting membrane potential of arteries below 200μm was –45.83±1.8mV (3) compared to –49.58±1.12mV (3) in arteries of 250–300μm, -58.13±2.8mV (6) in arteries of 300–350μm and –66.63±6.4mV (3) in arteries of 450–500μm. An unpaired, two-tailed t-test, showed significant difference between vessels of <200μm and 300-350μm (p=0.02) and between vessels sizes <200μm and 400-450μm (p=0.03). For the arteries investigated, myogenic constriction only occurred in vessels with a mean resting membrane potential of –45.83mV at the sub-myogenic pressure of 20mmHg. In summary, our data suggest that resting membrane potential of rat mesenteric arterial smooth muscle cells does correlated with passive (resting) arterial diameter.