1. We have identified putative histaminergic neurons in the central nervous system of Aplysia californica by light-microscopic autoradiography after uptake of [3H]histamine and by immunohistochemistry with the use of an antibody specific for histamine. 2. In the cerebral ganglion cells previously shown to contain histamine (C2 and 2 large neighboring cells in the E cluster and a group of smaller cells in the L cluster) were identified both by uptake of [3H]histamine and by histamine immunoreactivity. The identification of C2 was confirmed by experiments in which individual C2s were characterized electrophysiologically and injected with Lucifer yellow before processing for immunohistochemistry. The giant serotonergic neuron did not take up [3H]histamine and was not immunoreactive. 3. In the abdominal ganglion two clusters of cells--one in the left hemiganglion and the other in the right--took up [3H]histamine and were histamine immunoreactive. These clusters are located in the regions occupied by the 30 identified respiratory interneurons, R25 and L25. Individual cells in the R25 and L25 clusters were identified electrophysiologically, marked by injection of Lucifer yellow, and processed for immunocytochemistry. Eleven of the 30 L25 cells examined (from 7 ganglia) and 2 of the 25 R25 cells (from 6 ganglia) that had been marked with Lucifer yellow were also histamine immunoreactive. 4. Also in the abdominal ganglion, identified cells in the L32 cluster were not histamine immunoreactive and did not take up [3H]histamine. These interneurons, which mediate presynaptic inhibition, had previously been considered histaminergic. Neurons in the ganglion known to use transmitters other than histamine (L10, R2, RB cells, and bag cells) were not histamine immunoreactive.(ABSTRACT TRUNCATED AT 250 WORDS)