1. We studied the activation properties and assessed the functional role of the inwardly rectifying potassium conductance (GK.IR) in acutely isolated retinal horizontal cells (HCs) with the use of the whole cell patch-clamp technique. 2. The potassium current mediated by GK.IR was isolated by the use of Cs+ or Ba2+ ions. This current was outward, although relatively small in amplitude, in the voltage range between the potassium equilibrium potential (EK) and 50-60 mV more positive. The current reversed its polarity at EK and became inward at potentials more negative than EK. When HCs were bathed in normal Ringer (EK = -90 mV), GK.IR began to active at about -30 mV, was 30-40% activated at the resting potential (-70 to -80 mV) and about fully activated at -130 mV. Thus a significant portion of the activation range of GK.IR overlaps the HC physiological response range (-20 to -80 mV). 3. GK.IR has a dramatic effect on the kinetics of membrane polarization. Blocking GK.IR with Cs+ or Ba2+ significantly slowed the rate of membrane hyperpolarization in response to a hyperpolarizing current ramp over the HC physiological response range. Blocking GK.IR also dramatically slowed the onset rate of a simulated light response generated by a brief break in a sustained glutamate puff. 4. These results suggest that GK.IR can enhance the temporal resolution of the HC by accelerating the onset rate of the hyperpolarizing light response.