The effects of iontophoretically applied thyrotropin-releasing hormone (TRH) on cat retinal brisk-sustained(X) and brisk-transient (Y) ganglion cells were studied in the intact eye in vivo. Under photopic illumination we found a differential action of TRH on on- and off-centre cells: the maintained activity and light response were suppressed in on-centre cells and enhanced in off-centre cells. This was true for both brisk-sustained(X) and brisk-transient(Y) cells. In contrast, TRH did not influence the ganglion cell discharge under scotopic stimulus conditions. These results indicate that TRH acts on neurons presynaptic to ganglion cells and these neurons are only active under photopic conditions. We suggest that a possible functional role of this specific action of TRH is in light adaptation.