The effects of bradykinin (BK) and histamine on intracellular Ca²⁺ ([Ca²⁺]i) were studied in fura-2-loaded guinea-pig tracheal smooth muscle cells in culture. BK, at 10 nM, and histamine, at 100 μM, induced a rise in [Ca²⁺]i which was inhibited by the B2 antagonist Hoe 140 and by the H1 antagonist triprolidine, respectively. This rise in [Ca²⁺]_i is biphasic, consisting of a rapid transient phase followed by a sustained phase. The transient phase, induced by either BK or histamine, was strongly inhibited by thapsigargin, a microsomal Ca²⁺-ATPase inhibitor, usually used to deplete certain intracellular Ca²⁺-stores. Ni²⁺ (4 mM) did not affect the transient phase but abolished the sustained phase when cells were stimulated by BK, further supporting the fact that the transient phase was only dependent on intracellular Ca²⁺ pools. The sustained phase was partially (for BK) and completely (for histamine) inhibited by 30 μM Mn²⁺. This effect could be completely reversed by the addition of DTPA, a cell-impermeant chelator of Mn²⁺, indicating that the Mn²⁺ exerted its effect extracellularly. The presence of 1 mM probenecid (an inhibitor of a membrane organic anion transporter that extrudes fura-2) drastically inhibited the sustained phase by more than 77010 for BK and 88010 for histamine. Our results suggest that the effects of BK and histamine on airway smooth muscle cells are mediated via bradykinin B2 receptors and histamine H1 receptors, respectively whose activation allows the rapid transient rise in [Ca²⁺]i from thapsigargin-sensitive intracellular Ca²⁺ pools. The sustained phase is proposed to be drastically influenced by an acceleration of fura-2 extrusion during the increase of [Ca²⁺]_i via a probenecid-sensitive mechanism.