The same cells in both the intact retina and retinal cultures of the rabbit retina take up exogenous serotonin or 5,7-dihydroxytryptamine. Both substances can be localised by immunohistochemistry using a monoclonal antibody directed against serotonin. The 5,7-dihydroxytryptamine can also be revealed directly in both fixed and living tissues using appropriate U.V. light. Some of the cells in the intact retina and retinal cultures which take up 5,7-dihydroxytryptamine also stain positively for the immunohistochemical localisation of gamma-aminobutyric acid (GABA). Cell counts in the cultures show that 80% of all 5,7-dihydroxytryptamine-positive cells stain positively for GABA, while 20% of the GABA-immunoreactive cells also accumulate 5,7-dihydroxytryptamine. These results demonstrate unambiguously that a subpopulation of GABA cells has the capacity to take up exogenous 5,7-dihydroxytryptamine and therefore utilise GABA and, probably, serotonin. Findings in favour of the 'co-occurrence' of GABA and serotonin come from studies where tissues were exposed to radioactive GABA and unlabelled serotonin so that the dual localisation of these substances by autoradiography and immunohistochemistry could be followed. It could be shown both in cultures and in intact retinal pieces that certain cells take up GABA and serotonin while other cells take up either GABA or serotonin.