The endogenous amino acid content of intact retinas and isolated horizontal cells was measured by high performance liquid chromatography (HPLC). In both of these preparations, the concentration of gamma-aminobutyric acid (GABA) was approximately 2 mM. The endogenous GABA of the isolated horizontal cells was specifically released upon depolarization of the cells by elevated external K+ concentrations. This K+ induced release of endogenous GABA has two components. There is a saturable, Ca2+-dependent portion which predominates with small depolarizations, and a Ca2+-independent fraction which is linearly dependent upon the membrane potential of the horizontal cells. The data presented suggest the possibility of a dual scheme for the release of GABA from goldfish horizontal cells. The first is a Ca2+-dependent, conventional synaptic release; and the second a Na+-driven, carrier mediated release, which is linearly dependent upon the cellular membrane potential and functions in the absence of extracellular Ca2+.