Parvalbumin-immunoreactive structures in the fascia dentata and Ammon's horn of the adult human brain were studied using the avidin-biotin-peroxidase technique. Thin fibres (probably axons) were found to form dense networks throughout the cellular layers. Parvalbumin immunoreactivity is observed in even distal portions of nerve cell processes. The excellent quality of the immunoreaction renders the distinction of a large number of possible neuronal types. All parvalbumin-immunoreactive neurons belong to the class of non-granule cells in the fascia dentata and non-pyramidal neurons in Ammon's horn. The fascia dentata harbours four types of neurons in the molecular layer, one type within the granule cell layer and four types in the plexiform layer. The frequently described basket cells are contained in the group of immunoreactive non-granule cells in the plexiform layer. In field CA4 two neuronal types can be distinguished. Field CA3 reveals a slender cell type in the stratum radiatum, three types in the pyramidal cell layer and three types in the stratum oriens. In field CA2 three neuronal types can be differentiated in the stratum pyramidale. The extended field CA1 is endowed with two types of nerve cells within the stratum moleculare, two types in the stratum radiatum, five neuronal types in the stratum pyramidale, and one spindle-shaped type in the stratum oriens. The morphological features of parvalbumin-immunoreactive neuronal types in the adult human brain are compared with those found in Golgi-studies of mostly young animals or in labelling experiments. This study serves as a basis for further analyzes involving specific diseases such as Alzheimer's disease or epilepsy, where it needs to be clarified to which extent certain neuronal types are afflicted.