1. The roles of Na and Ca ions in stimulus-secretion coupling were analysed in the isolated and perfused rat pancreas.2. Partial replacement of NaCl with LiCl produced a diminution in both amylase output and pancreatic juice flow which were induced by 5 m-u. CCK-PZ/ml., and almost normal responses were usually regained immediately after the reintroduction of a standard concentration of NaCl. Nearly total replacement of NaCl with LiCl caused an almost complete inhibition of the responses, although 25 mM-NaHCO(3) and 1 mM-NaH(2)PO(4) were still present, and only partial recovery was obtained after the re-introduction of a standard concentration of NaCl.3. A quantitative relationship was found between the amount of amylase released by CCK-PZ and the [Na(+)](o) over the range 26-157 mM in the presence of 2.5 mM-Ca. A similar relationship was also observed when [Ca(2+)](o) was decreased to 1.0 mM, but the responses were reduced to about one half of those observed with 2.5 mM-Ca.4. The most satisfactory theory which explains the cellular mechanism of CCK-PZ-induced amylase output, and which fits the experimental data, requires the dominant activity of a complex composed of a carrier molecule bearing one Ca and four Na molecules, if there is no interaction between Na(+) and Li(+).5. A quantitative relationship was also found between the amount of pancreatic juice flow stimulated by CCK-PZ and [Na(+)](o), over the range 26-157 mM, in the presence of 1.0 or 2.5 mM-Ca.6. A similar quantitative relationship was found between the amount of amylase released by Ca-ionophore A23187 and [Na(+)](o) in the presence of 2.5 or 5.0 mM-Ca. The most satisfactory theory which fits this experimental data also requires the dominant activity of a complex composed of a carrier molecule bearing one Ca and four Na molecules, if there is no interaction between Na(+) and Li(+).