In vivo intracellular recording and labeling in kainate-induced epileptic rats was used to address questions about granule cell axon reorganization in temporal lobe epilepsy. Individually labeled granule cells were reconstructed three dimensionally and in their entirety. Compared with controls, granule cells in epileptic rats had longer average axon length per cell; the difference was significant in all strata of the dentate gyrus including the hilus. In epileptic rats, at least one-third of the granule cells extended an aberrant axon collateral into the molecular layer. Axon projections into the molecular layer had an average summed length of 1 mm per cell and spanned 600 microm of the septotemporal axis of the hippocampus-a distance within the normal span of granule cell axon collaterals. These findings in vivo confirm results from previous in vitro studies. Surprisingly, 12% of the granule cells in epileptic rats, and none in controls, extended a basal dendrite into the hilus, providing another route for recurrent excitation. Consistent with recurrent excitation, many granule cells (56%) in epileptic rats displayed a long-latency depolarization superimposed on a normal inhibitory postsynaptic potential. These findings demonstrate changes, occurring at the single-cell level after an epileptogenic hippocampal injury, that could result in novel, local, recurrent circuits.