The use of antisera directed against conjugates of histamine and serotonin has revealed the locations of neurons labeling for these transmitters in the nervous system of barnacles. Photoreceptors label for histamine but not serotonin and also satisfy a number of other criteria indicating that histamine is their neurotransmitter. Photoreceptors also take up radioactively labeled histamine but not serotonin. Within the barnacle's brain no somata are consistently found that label with antiserum against histamine, but one to three pairs of small cells, depending on species, label with antiserum against serotonin. The most impressive serotonin-like immunoreactivity in the brain, however, is in a pair of large fibers ascending through the circumesophageal connectives and ramifying extensively. Within the ventral ganglion, the only other ganglion in the barnacle, ten pairs of cells label with antiserum against histamine. These neurons are confined to the posterior portion of the ganglion but ramify extensively throughout the ganglion. Antiserum against serotonin labels about 15 cell pairs, depending on species, located throughout the ganglion. The positions of the arbors of many of these cells suggest that these amines have a role in modulating either the motor pathways underlying feeding or the visual pathways responsible for the detection of shadows.