The possible coexistence of calbindin D28k with parvalbumin and of calbindin D28k with cholecystokinin was studied in nonpyramidal cells of the rat dorsal hippocampal formation. Neighbouring Vibratome sections were immunostained either for calbindin D28k and parvalbumin or for calbindin D28k and cholecystokinin. The cells, halved during sectioning, were identified in both sections immunostained for different antigens. The coexistence of calbindin D28k and parvalbumin in the same neuron was rare throughout the hippocampal formation with the exception of stratum oriens of the CA1 region, where 9.6% of the parvalbumin-immunoreactive cells also contained calbindin D28k. In stratum radiatum of the CA3 region, calbindin D28k and cholecystokinin coexisted in 12.5% and 21.2% of the calbindin D28k and cholecystokinin-immunoreactive cells, respectively. In other regions of the hippocampal formation, the two markers coexisted in less than 5% of the cells of either type. The present results demonstrate that calbindin D28k-, parvalbumin- and cholecystokinin-containing nonpyramidal cells represent largely nonoverlapping cell populations and may thus be involved in different inhibitory circuits.