A-type currents are voltage-dependent,Ca²⁺-insensitive potassium currents that undergo rapid activation and inactivation. A-type currents have been identified and characterised in several types of vascular smooth muscle, but little is known about their general properties in the microvasculature. In the present study we have used the whole-cell perforated patch-clamp technique to identify and characterise the A-type current in arteriolar myocytes from the rat retina. Sprague Dawley rats (200-300g) were anaesthetised using CO₂ and killed by cervical dislocation. Microvessels were mechanically dispersed from fresh retinae using a fire-polished Pasteur pipette and ionic currents were recorded from myocytes whilst still embedded within their parent arterioles. Just prior to patch clamping, vessels were digested with an enzyme cocktail of collagenase 1A and protease type XIV to remove surface basal lamina and to electrically uncouple endothelial cells from overlying arteriolar smooth muscle cells. Patch electrodes (1-2MΩ) were K⁺-based (138 mM) and contained 600 μg ml^-1 amphotericin B. Data are expressed as mean ± SEM. K⁺ currents were activated by depolarising steps from -80 mV to +100 mV in 10 mV increments from a holding potential of -80 mV. A dominant, non-inactivating Ca²⁺-activated K⁺ current was elicited by depolarisation. Inhibition of this current by 100 nM Penitrem A revealed a rapidly inactivating K⁺ current which resembled an A-type current. This transient current activated at a potential of -60 mV with peak current densities averaging 23.4 ± 3.9 pA/pF at +60 mV (n=10). The voltage of half-inactivation was -28.9 mV and the time constant for recovery from inactivation at +60 mV was 105.5 ms (n=10). The A-type current was relatively insensitive to 1 mM tetraethylammonium (11.1 ± 3.8% inhibition at +60 mV; n=4), but was substantially blocked by 10 mM 4-aminopyridine (68.4 ± 4.4% at +60 mV; n=9). The specific Kv4 channel blockers, heteropodatoxin (1 μM) and phrixotoxin (500 nM) failed to reduce the A-type current (n=3 and 2, respectively). These results demonstrate that an A-type current is present in retinal arteriolar myocytes, but that in contrast to those identified in many other types of smooth muscle, the current does not appear to be mediated by the Kv4 family of K⁺ channels. The half-inactivation voltage for A-type current in retinal arteriolar myocytes is ~ 20 mV more positive than values recorded for other types of smooth muscle, further supporting the idea that the molecular composition of the underlying channels is different.