The correlation between the receptive field centre and the dendritic tree of individual brisk transient, or alpha, ganglion cells in the cat retina was investigated by a combination of physiological and anatomical techniques. The sizes of receptive field centres of brisk transient (Y) cells were measured with a flickering spot of light. Contour maps and response (or sensitivity) profiles were measured at mesopic and scotopic backgrounds. Recording positions on the retina and nearby blood vessels were back-projected onto the receptive field plots on the tangent screen. After recording, whole amount preparations of the retinae were stained by a reduced silver method to stain all alpha cells together with their dendritic trees. By comparing the landmarks on the screen plot with those of the whole mount it was possible to identify the recorded cells in the preparation and to study their morphology. The dendritic tree of an alpha cell determines the position, size and shape of its receptive field centre. The mesopic receptive field centres were found to be a factor of 1.4 +/- 0.13 larger than their respective dendritic fields. It is suggested that the dendritic fields of presynaptic neurones (bipolar and amacrine cell processes) add to the ganglion cell dendritic tree to produce the larger centre summating area.