1. Nitric oxide (NO) relaxes vascular smooth muscle (VSM) by mechanisms which are not fully understood. One possibility is that NO hyperpolarizes membranes, thereby diminishing Ca2+ entry through voltage-dependent Ca2+ channels. In the current study, the effects of NO on membrane potential of rabbit mesenteric arteries were recorded using intracellular microelectrodes. 2. NO, released by 3-morpholinosydnonimine (SIN-1, 3 microM), reversibly hyperpolarized arteries by -9.5 +/- 4.0 mV (means +/- S.D., n = 97) from a resting membrane potential of -53.1 +/- 5.7 mV. The hyperpolarization was blocked by oxyhaemoglobin (20 microM), and only occurred in arteries pre-treated with N omega-nitro-L-arginine (100 microM) or denuded of endothelium. 3. The effect of SIN-1 was concentration dependent (EC50 approximately 0.4 microM) and its dose response was shifted to the left by zaprinast (100 microM), an inhibitor of cGMP-specific phosphodiesterases. 4. The hyperpolarization due to SIN-1 was modified by changes in extracellular K+ concentration, but not by changes in Ca2+, Na+ or Cl-. The hyperpolarization was blocked by glibenclamide (IC50 approximately 0.15 microM), but not by apamin (3-300 nM), barium (5-150 microM), tetraethylammonium (0.1-10 mM), or 4-aminopyridine (5-500 microM). The hyperpolarization due to lemakalim (0.03-3 microM), an activator of ATP-sensitive potassium channels (KATP), displayed the same sensitivities to these K+ channel blocking agents, whereas the endothelium-derived hyperpolarizing factor, triggered by the addition of acetylcholine (3 microM), caused a hyperpolarization (-15.3 +/- 6.2 mV) that was blocked by apamin, but not by any other agent. 5. These results suggest that NO hyperpolarizes VSM in rabbit mesenteric arteries by activating KATP channels, with the accumulation of cGMP as an intermediate step.