The distribution of dynorphin (DYN), one of its binding sites (kappa 1 receptor) and their relationship to neuronal loss and granule cell hyperexcitability was examined in hippocampi from patients with temporal lobe epilepsy (TLE). In hippocampi that were not the seizure focus (mass associated temporal lobe epilepsy, MaTLE; and paradoxical temporal lobe epilepsy, PTLE) DYN-like immunoreactivity was localized in the dentate granule cells and their mossy fiber terminals within the hilus and area CA3. In hippocampi that were the seizure focus (MTLE), 89% showed an additional band of immunoreactivity confined to the inner molecular layer (IML) of the dentate gyrus, representing recurrent mossy fiber collaterals. In 11% of MTLE patients no staining was found in the IML (MTLE/DYN-). The MTLE/DYN- hippocampi were also characterized by a significantly lower degree of cell loss than in MTLE hippocampi in the dentate granule cell layer, the hilus and CA3. Both MTLE and MTLE/DYN- hippocampi showed evoked epileptiform bursting in granule cells while MTLE showed greater polysynaptic EPSPs and spontaneous excitatory activity. Thus granule cell recurrent collateral sprouting may account for only some aspects of hyperexcitability. In 30% of the MTLE group, hilar neurons of a variety of morphological types expressed DYN immunoreactivity in their somata and dendrites. The density of [3H]U69,593 binding sites in MaTLE and PTLE patients was highest in areas CA1 and the subiculum-regions having little or no DYN-staining. In the dentate molecular layer, hilus and CA3--regions with the most DYN immunoreactivity--there was a low density of ligand binding. The significance of this transmitter/receptor mismatch is yet unknown.