Rat pancreas acinar cells contain 5-hydroxytryptamine (5-HT; 10.86 +/- 2.52 ng/i.u. amylase), all of which can be accounted for by the 5-HT recovered from the zymogen granule fraction of these cells (10.70 +/- 3.06 ng/i.u. amylase). When incubated with [14C]5-HT dispersed acinar cells take up the amine and concentrate it in zymogen granules. These cells will also take up [14C]5-HTP (5-hydroxytryptophan), decarboxylate it and store the [14C]5-HT so produced in zymogen granules. 5-HTP itself is not taken up by the granules. 5-HT is incorporated into zymogen granules early in their formation; no amine is accumulated by mature zymogen granules and the amine within the mature granule is not exchangeable with extragranular amine. When dispersed acinar cells pre-labelled with [14C]5-HT and [3H]leucine are stimulated with caerulein, there is a synchronous increase in amylase activity and secretion of [14C]5-HT and [3H]protein; the ratios of [3H]protein/[14C]5-HT in zymogen granules and in the secretory products are identical. Pancreas acinar cells take up L-DOPA, decarboxylate it and store the dopamine produced in zymogen granules but the dopamine is not retained by the granules (t1/2 approximately equal to 90 min) and dopamine secretion from cells exposed to caerulein could not be demonstrated. It is concluded that 5-HT is a normal component of rat pancreas acinar cell zymogen granule. The granular amine has a turnover rate similar to that of granular protein and is released when the cells are stimulated to secrete protein. All the 5-HT released from the cell originates in zymogen granules.