In previous work we described 4 types of visual response among tectal cells of the zebrafish. Cells of one class, type I, have no spontaneous activity, but respond phasically at ON and OFF. Their responses to moving edges, to stimuli that grow in size, and to stimuli equal in size and shape to the whole receptive field (RF) suggest that these cells may receive inhibitory input from near neighbor cells of the same type in the tectum, as well as excitatory input from retinal fibers. In order to further investigate this hypothesis we have studied the effects of drugs on physiological properties of type I cells recorded in the stratum periventriculare layer of the zebrafish tectum. Small (10-50 nl) injections of drugs were made in the tectum while recording 100-500 microns away with extracellular microelectrodes. Both picrotoxin and bicuculline produce the following effects: (1) onset of spontaneous bursting multiunit activity. This noise can be recorded at all depths within the tectum; (2) abolition of the second postsynaptic wave of the optic nerve shock field potential and the current source responsible for it, which occurs in the upper tectal layers at 8 ms latency. This probably represents the secondary activation of inhibitory synapses in those layers; (3) alteration of visual response properties of individual type I tectal cells. The duration of response to small flashing spots and to stimuli that grow in size both increase significantly. Responses to moving edges, which normally occur mostly as the significantly. Responses to moving edges, which normally occur mostly as the edge is crossing the RF border, become extended to encompass the entire RF. Finally, the cells show reduced negative spatial summation following drug injection. All of these effects are fully reversible with time after injection as the drugs wash out. Control injections (of teleost Ringer's solution, 100 mM HCl, 165 mM NaCl, and strychnine 2 mM or 5 mM) do not elicit any of these effects. The results reported here are consistent with the hypothesis that tectal type I cells receive a delayed inhibitory input, probably via GABA synapses, which determines major properties of the visual response.