The spatial distribution of transient K current, IA, was studied using a combination of patch-clamp and whole-cell voltage-clamp techniques. The average IA current density in somatic patches is 0.64 times the current density in the entire axotomized cell body, a finding which suggests that the axon hillock or initial segment of the axon has a higher concentration of IA channels than much of soma. The highest density of active channels during the peak IA is 1/micron2 at a membrane voltage of -20 mV. There is no evidence for a gradient in the distribution of IA channels in the cell body, but the channels are not evenly distributed. The variability in the number of channels per patch for multiple patches on the same neuron is much higher than expected for a random distribution. Statistical analysis of the data yields a coefficient of dispersion of 8.1, a value indicating a high degree of clustering. The utility of this statistic for evaluating channel distributions is discussed. Several lines of evidence suggest that the upper limit for the area of IA channel clusters is approximately 250 micron2. Single-channel currents attributed to IA were recorded in the cell-attached configuration. The voltage dependence of channel opening and inactivation are the same as measured in whole-cell voltage-clamp experiments. The single-channel conductance is about 9 pS in normal saline. Patches 9-30 micron2 in areas that contain IA channels are often devoid of other K channel types, suggesting that IA channels can occur in isochannel clusters. IA inactivation follows an exponential time course in all of the neurons examined, but the time constant of inactivation ranges from 25 to 560 msec in different cells. The voltage dependence of activation and inactivation and the reversal potential of the current are approximately the same in all cells. When multiple patches on the same neuron are studied, it is found that IA inactivates exponentially with approximately the same time constant in each patch, regardless of patch area. The data suggest that each neuron expresses predominantly, and perhaps exclusively, a single type of IA channel with distinct kinetic properties. The wide range of IA inactivation time constants observed in different cell suggests that a large number of channel types are available for expression. Possible mechanisms for generating diversity in channel types are discussed.