Behavioral and electrophysiological measures were used to elucidate the retinal modulation of oculomotor control in the turtle. Eye movements were recorded following intravitreal applications of 2-amino-4-phosphonobutyrate (APB) and the GABA antagonists picrotoxin and bicuculline. Visual responses of single basal optic nucleus (BON) neurons of the accessory optic system were studied in parallel experiments. The effectiveness of APB, a glutamate analog thought to act selectively on the retinal ON pathway, was assessed independently by recording electroretinograms or ganglion cell activity. Injections of APB into the turtle's eye reduced or blocked the injected eye's ability to drive horizontal optokinetic nystagmus, as also observed in rabbit and cat (Knapp et al., 1988; Yucel et al., 1989). Single-unit recordings from the BON during APB superfusion (50–200 μM APB) of the contralateral retina demonstrated that these cells, which are direction-sensitive and respond to the offset of light flashes, have their responses to moving stimuli blocked by APB. During the APB effect, GABA antagonists were applied to the same eye. Although moderate doses of APB were sufficient to block optokinetic or BON light responses, the addition of GABA blockers still elicited a spontaneous temporal-to-nasal nystagmus (Ariel, 1989) or visually responsive yet direction-insensitive responses from BON cells (Schuerger et al., 1990). These results are discussed in terms of the retinal output to pathways involved in oculomotor control of optokinetic nystagmus. © 1991, Cambridge University Press. All rights reserved.