Previous reports have suggested a role played by peripheral benzodiazepine receptors (PBZ) in the regulation of calcium. The aid of this investigation was to evaluate the effects of Ro 5-4864 (PBZ agonist) and PK11195 (PBZ antagonist) in guinea pig trachea and their possible role in the regulation of calcium. Ro 5-4864 and PK 11195 showed spasmolytic effects on the contraction curve induced by acetylcholine. Moreover, Ro 5-4864, PK 11195 and diltiazem (a Ca2+ -antagonistic drug) shifted to the right the contraction curves induced by acetylcholine, potassium chloride (KCl), calcium chloride (CaCl2), or by Bay K 8644. These results demonstrate that the benzodiazepines and their antagonists may exhibit Ca2+ -antagonistic activity. Ro 5-4864 and PK 11195 may exert this effect at a site that is a component of, or is associated with, voltage operated calcium channels (VOC).