The non-pyramidal cells of the hippocampus are heterogeneous with respect to their morphology, peptide content, physiological properties, and postsynaptic targets. Here we demonstrate that the content of peptides (cholecystokinin, somatostatin) and calcium-binding proteins (parvalbumin and calbindin) of non-pyramidal cells is not related to a characteristic fine structure or synaptic input. Varying numbers of GABA-negative and GABA-positive input synapses of non-pyramidal cells indicate that these neurons are differently integrated in inhibitory and disinhibitory circuits.