Purpose: To investigate the contribution to the photopic negative response (PhNR) of the electroretinogram (ERG) by retinal ganglion cells (RGCs). The PhNR was assessed longitudinally following optic nerve transection (ONTx). Methods: Photopic ERGs were recorded from each eye of an anesthetized (ketamine/xylazine, 60 mg/kg and 5 mg/kg) Brown Norway rat using custom made electrodes (PT-IR Tef., A-M System Inc). ERGs were elicited using green Ganzfeld flashes (11.38 scd/m(2), 22.76 cds/m(2)) and a rod suppressing green-background (40 cd/m(2)). PhNRs were compared before and after optic nerves were transected. Cresyl violet stained retinal flatmounts were used to estimate cell loss in the ganglion cell layer 3 and 15 weeks after optic nerve transection. The pharmacological effect of 1.3 muM intravitreal TTX on the PhNR was also evaluated. Results: There was a significant loss (p