The spiny lobster stomatogastric nervous system synthesizes dopamine and octopamine in vitro from exogenous [3H]tyrosine. Each amine accumulates with a specific distribution among 9 separately analyzed regions within the system. Synthesis of other catecholamines was not observed. [3H]Dopamine is found in nerves, ganglia, and identified commissural ganglion cell bodies in which catecholamine histofluorescence has been demonstrated. The biosynthetic and histochemical data together indicate that dopaminergic cells send axons from the commissural ganglia to the stomatogastric ganglion neuropil along the same pathway followed by fibers that activate the pylroic motor network. The results support the hypothesis that dopamine mediates activation of the pyloric system in vivo, as observed in vitro. [3H]Octopamine accumulates primarily in the commissural and stomatogastric ganglia, where it may modulate neuronal activity, but octopaminergic cells and release sites within the stomatogastric system have not been identified.