The effects of picrotoxin and strychnine on the centre surround types of ganglion cell (X, Y, sluggish sustained and sluggish transient with on or off centres, and colour coded) were studied in the rabbit retina. 2. Picrotoxin changed the centre surround balance in favour of the centre for Y cells and sluggish transient cells but not for X cells or sluggish sustained cells. 3. Inhibition by a moving radial grating was abolished by picrotoxin for off centre Y cells, but not for on centre Y cells. 4. Picrotoxin abolished the surround response for six on centre sustained cells. These were hybrid cells with conduction velocities and receptive field properties characteristic of more than one of the X, Y and sluggish categories. The surround was not abolished by picrotoxin for any of the cells which fell in the standard X, Y and sluggish categories. 5. Strychnine did not affect the centre surround balance substantially in any of the cells tested. Strychnine did effect the transients: in general strychnine shortened or abolished them, while picrotoxin made them larger.
