1. Potassium currents in chick cochlear hair cells were studied using whole-cell voltage clamp techniques. Cells were isolated from 200 microns-long segments of the apical half of the cochlea. In each segment, expression of potassium currents by cells positioned across the width ('inner-outer' hair cell axis) of the cochlea was examined. 2. A rapidly inactivating potassium current (IA) was found in some hair cells. At a membrane potential of -24 mV, IA activated to peak values within 7 +/- 1 ms and inactivated within 73 +/- 16 ms. The activation 'threshold' was around -50 mV and hyperpolarization more negative than -56 +/- 5 mV was required before significant removal of inactivation occurred (V 1/2 (half-inactivation potential) = -74 +/- 5 mV). The resting potential of cells with IA was -46 mV +/- 11 mV. This current was blocked by 4-aminopyridine with a Kd of 0.45 mM. 3. Cells that were isolated from the most apical tip of the cochlea expressed no IA. In areas more basal than 200 microns from the apex, the magnitude of IA correlated with cell morphology. In each area, the tallest hair cells (cells with the smallest ratio of apical surface diameter to length) had none of this current. Of the cells with IA, the shorter cells (larger ratio of apical surface diameter to length) had more of this current. 4. The magnitude of IA in a cell was dependent upon cross-cochlear position, and the relationship between IA and cell morphology was most probably a reflection of a differential distribution of cell shape across the cochlea. The tallest hair cells, occupying roughly the first 40% of the distance from the neural side of the basilar papilla, had no IA. Of the remaining cells, those nearer to the abneural edge expressed more IA, such that iso-magnitude lines ran approximately parallel to the long axis of the cochlea. 5. A delayed rectifier current (IK) and an inward rectifier current (IIR) were also differentially distributed among hair cells across the cochlea; however, their distribution differed from that of IA. IK and IIR were preferentially expressed by the taller hair cells, which were positioned nearer to the neural side of the cochlea. Ca(2+)-activated potassium current (IK(Ca)) did not vary systematically between cells of different shape or cross-cochlear position, and IK(Ca) could often be found in cells with IA.