The stimulus response characteristics (SRCs) of 33 phasic retinal ganglion cells were established on the basis of extracellular recordings in the optic tract of anaesthetized and paralyzed cats. All SRCs were best described with two straight lines in double logarithmic coordinates. The near threshold light intensity summation was found to be linear, on the average up to 4.8 times threshold. The cells' threshold, defined as smallest response outside the 95%-confidence interval of the spontaneous activity (SpA) distribution, is dependent on the slope of the gain near threshold (linear gain) and the standard deviation of the spontaneous activity (SpA-scatter) prior to stimulation. The slope of the double logarithmic relationship at higher intensities (non-linear gain) - corresponding to the exponent of the power function - increases with threshold intensity. The linear, near threshold gain was used to describe the retinotopic distribution of the cells' threshold- and suprathreshold sensitivity. This sensitivity is high in the center of the retina decreasing steadily towards the periphery. Threshold, as well as linear and non-linear gain are interdependent parts of the SRC, specific for each ganglion cell and, furthermore, the geometrical mean between threshold activity and the response activity at the intersection point of the two regression lines is constant around 30 imp/s, irrespective of the cell's range of operation. The entire course of the SRC can therefore be predicted on the basis of the SpA-scatter and threshold intensity. The homogeneous population of investigated Y-ganglion cells proved to be a set of cells with summation characteristics, changing systematically with threshold and the distance of the receptive field from area centralis.