Amino acid decarboxylation activity in dispersed rat pancreas acinar cells and fractions derived by differential centrifugation of homogenate of these cells was studied. The rate of decarboxylation was measured by determining the rate of production of the [3H]-amine from [3H]-amino acid or the rate of production of 14CO2 from the [14C]-carboxy-labelled amino acid. Only the hydroxylated amino acids L-dopa and 5-hydroxytryptophan are decarboxylated by intact dispersed pancreas acinar cells or cell homogenates at all pH values and amino acid concentrations tested. The decarboxylase activity is located exclusively in the cell cytosol. Each substrate competitively inhibits the decarboxylation of the other and the decarboxylation of each is inhibited by NSD-1055. The estimated Km and Vmax are, for L-dopa, 4.8 X 10(-5) M and 2.5 nmol/mg protein/min and for 5-hydroxytryptophan, 2.9 X 10(-5) M and 0.3 nmol/mg protein/min. The pH optimum for 5-hydroxytryptophan decarboxylation is from 7.0-8.5 while that for L-dopa is 7.0. It is concluded that pancreas acinar cells possess a single aromatic amino acid decarboxylase specific for the hydroxylated amino acids L-dopa and 5-hydroxytryptophan, and which is similar in all properties studied to the aromatic amino acid decarboxylase found in several other mammalian tissues.