The linearity of spatial summation within the receptive fields of Necturus retinal ganglion cells was examined with a counterphase-modulated split-field test stimulus. Modulation-locked histograms were constructed with the border at a variety of positions relative to the receptive field center. Most cells generated distinct, modulation-locked discharges when the border was 0.10 to 0.25 mm from the receptive field center. The discharges for more central border positions depended upon response type. Sustained-responding cells showed a sharp decrease in total response per cycle at the center position and increasing response for increasingly eccentric positions, i.e., nulling or X-like behavior. In contrast, the total response of transiently responding cells was largely independent of border position. For central positions of the border, these cells generated two bursts of impulses per cycle, i.e., frequency doubling or Y-like behavior.