The role of Ca ions in stimulus-secretion coupling has been analysed in the isolated and perfused rat pancreas. 2. The omission of [Ca2+]O diminished but did not abolish the release of amylase in response to continuous stimulation with 5 m-u. pancreozymin (Pz)/ml. The addition of Mn2+ (1-0 mM) to this Ca-deficient environment abolished the residual release of amylase. This was followed by a complete recovery of amylase output when the control [Ca2+]O was reestablished. 3. The addition of Mn2+ (1-0 mM) to the extracellular environment containing 2-5 mM-Ca2+ reversibly inhibited the Pz-induced release of amylase. 4. A kinetic scheme based on competition of Ca and Mn at a carrier in the acinar cell membrane could quantitatively explain the effects of Ca and Mn upon the Pz-induced amylase release. 5. These results support the view that the Ca2+ influx into the acinar cells is the major contributor to the rise in [Ca2+]i which, in turn, mediates the processes in the stimulus-secretion coupling in the exocrine pancreas, and suggest that the mode of Ca influx is a facilitated diffusion.