AbstractThe pattern of adrenergic innervation to melanophores in split tail fins of the goby,Tridentiger obscurus, and the guppy,Lebistes reticulatus, was investigated using3H‐norepinephrine (3H‐NE) by light microscopic autoradiography. In goby fins, comparatively thick nerve bundles labeled with3H‐NE ran approximately parallel to fin rays. Thin varicose fibers which branched out from the thick bundles formed rough plexuses around individual melanophores. In guppy fins, no thick nerve bundles could be observed, but numerous thin varicose fibers ran between fin rays, forming dense nerve plexuses on individual melanophores. In setioned preparations of guppy fins, the3H‐labeled neural elements were observed to be in close contact with both surfaces of the cells (i.e., those facing the epithelial cell layer and the split side of fins), while in goby fins, the neural elements could be found only on the epithelial surface of the cells. Usually, melanophores in freshly isolated guppy fins responded with a full aggregation of pigment to isotonic KCl and maintained the state during the repetitive administration of the medium. Meanwhile, the degree of pigment‐aggregation response of goby melanophores to isotonic KCl decreased considerably during the treatment. The difference in the mode of melanophore response to K+between two fish species was considered to be due to the difference in the density of neural elements around each melanophore and, probably as a consequence, in the amount of released neurotransmiter which acts on