In the rat piriform cortex, double-fluorescence immunocytochemistry was used to study the coexistence of two calcium binding proteins, parvalbumin and calbindinD28k, and GABA. Layer I cells were immunoreactive only for GABA. In layer II a small number of cells were immunoreactive for all three substances. A few other cells contained only parvalbumin and GABA immunoreactivity. In layer III a subpopulation of calbindinD28k immunoreactive cells (about 60%) were also immunoreactive for both parvalbumin and GABA. The remainder showed only calbindinD28k and GABA immunoreactivity. Many immunoreactive terminals, which surrounded non-immunoreactive cell somata to form basket endings in layers II and III, were immunoreactive for all three substances. Their major source may be the non-pyramidal neurons in layer III, which contained the three chemical substances.