1. Receptive fields of centre surround cells in the rabbit retina were investigated. There is a clear distinction between cells with sluggish responses, low spontaneous activity and slow conduction velocity (centre surround sluggish cells) and cells with brisk responses, higher spontaneous activity and faster conduction velocity (X and Y cells). The sluggish cells can be divided into sustained and transient types. X and Y cells can be distinguished from each other by their responses to a moving linear grating, a large rapidly moving object and whether or not there is a response to the alternation of certain stimuli. Some times the response to a rotating radial grating, the rate of spontaneous activity, and whether or not the response to spots and annuli was sustained or transient could also be used to distinguish these two types. The antidromic latency from electrical stimulation of the optic chiasm and the periphery effect did not distinguish X from Y. 2. Eleven colour coded units were investigated. They all gave on responses to blue light in the centre of their receptive field and off responses to green light in the periphery of their receptive field. The blue pigment had a spectral sensitivity peaking at about 465 nm. The other pigment peaked near 500 nm, like the rods but gave a response at high mesopic and probably photopic levels. In some cases there was evidence for excitatory input from the green receptors to the centre of the receptive field. All the colour coded cells had rapidly conducting axons and were on centre X cells by all criteria. 3. Eighty-five cells various types other than colour coded were tested for their thresholds at 420 nm and 590 nm. In all cases the results were explained by a pigment peaking close to 500 nm, even at high mesopic and low photopic levels, which suggests the existence of cones with a cyan pigment in them. 4. Conduction latency from stimulation at the optic chiasm was measured for cells with centre surround receptive fields and cells with more complex receptive fields. Both 'on-off' and 'on' directionally sensitive cells have short conduction latencies, overlapping X and Y cells. Orientation selective cells and local edge detectors have long conduction latencies, overlapping centre surround sluggish cells. The sample of uniformity detectors was too small to characterize...