Ethambutol, a tuberculostatic drug, induces red-green colour vision defects in man and goldfish. The ethambutol-induced red-green colour vision defect in goldfish was argued to originate in the retina because after ethambutol application: (1) inhibitive interactions in red-green (double) opponent ganglion cells are lost [Van Dijk & Spekreijse, 1982 (Investigative Ophthalmology and Visual Science, 24, 128-133); Wietsma & Spekreijse, 1992 (Investigative Ophthalmology and Visual Science Suppl., 33, 1032)] and (2) the depolarizing responses to red light in the biphasic horizontal cells are reduced. To account for these findings Spekreijse, Wietsma and Neumeyer [(1991) Vision Research, 31, 551-562] suggested that ethambutol induced dark adaptation in the retina. In this paper the dark adaptation hypothesis is tested with the following results: (1) ethambutol changes only transiently the receptive field size and spectral sensitivity of horizontal cells; (2) the spectral characteristics of horizontal cells do not change in long-term ethambutol-treated goldfish; (3) formation of spinules on horizontal cell dendrites in cone terminals, a parameter for light adaptation, remains unaffected. Therefore we conclude that ethambutol does not induce functional dark adaptation of horizontal cells and that the ethambutol-induced red-green colour vision deficiency does not originate in the horizontal cell layers.