The medial septal diagonal band complex (MS/DB), made up of cholinergic and GABAergic neurons, plays an important role in the generation of the hippocampal theta rhythm. A GABAergic neuron type in the MS/DB that has fast spiking properties was shown previously to contain parvalbumin immunoreactivity and to form axosomatic connections with unidentified somata. The aim in the current study was to determine the neurochemical identities of these target neurons. In slices and in perfused-fixed brain, staining for parvalbumin immunoreactivity first of all revealed the presence of two types of parvalbumin-positive somata in the MS/DB: medially located neurons with parvalbumin-positive basket-like terminals on them, and more laterally located neurons with fewer parvalbumin-positive contacts on them. In MS/DB slices, the terminals of fast spiking neurons filled with biocytin correspondingly made either numerous contacts that surrounded the parvalbumin-positive cell body in basket-like formation, or 1-5 contacts on a localized patch of the soma. These contacts were shown by electron microscopy to form synaptic junctions. No terminals of biocytin-filled fast spiking neurons were observed on cholinergic neurons, and dual staining in perfused-fixed brain did not reveal the presence of parvalbumin-containing terminals on cholinergic somata. Our results suggest therefore that there are two subtypes of parvalbumin-containing neuron in the MS/DB, and that these are interconnected via axosomatic synapses. The contrasting topographical organization of the two types of parvalbumin-containing neuron suggests that they may receive different types of afferent input, but this will require substantiation in future studies. We propose that generation of rhythmic activity in the MS/DB is controlled by contrasting contributions from two types of parvalbumin-positive neuron, and that the role of the cholinergic neuron is modulatory.