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Long-term therapy with quensyl is known to cause neurodegenerative changes in the retina. In our present research we study acute toxic effects of quensyl on the a-wave response of electroretinogram (ERG) of isolated superfused bovine retinas.Isolated bovine retinae were mounted in a temperature-controlled recording chamber. After light stimulation electric field potentials were recorded as a transretinal potential using Ag/AgCl-electrodes. Isolated bovine retinas were perfused with phosphate buffered saline (PBS) containing 1mM L-aspartate to block further synaptic transmission in order to record the effects of quensyl on photoreceptors. We tested the low and high light intensities of 100 mlux and 10 lux. After reaching a stable ERG amplitude, quensyl (190 μM, 570 μM or 1,9 mM) was added to the perfusing solution. After 90 min quensyl was washed out for 90 min with PBS containing 1mM L-aspartate. Changes in a-wave amplitude were calculated and plotted.190 μM quensyl showed a 1,3 fold stimulation of the a-wave amplitude at 100 mlux, the effect at 10 lux was not significant. 570 μM quensyl reduced the a-wave amplitude by 3-folds independent of light-intensity. The inhibition was good reversible by washing with PBS containing 1mM L-aspartate only at low light intensity (2 folds), there was almost no recovery at 10 lux. 1,9 mM quensyl showed a massive depression of a-wave amplitude (6 to 7 folds) and no wash out effect over 90 minutes at both light intensities.Quensyl has a toxic effect on photoreceptors, even with slight increase of concentration showing a huge progression of inhibition and reduction of recovery. An exact dosage of quensyl is of great importance to avoid an irreversible neuronal damage.