1. The spatial properties of the small-field luminosity horizontal cell (L2-HC) were investigated in the retina of the turtle by using circular or annular light stimuli of varying dimensions. 2. The amplitude of the hyperpolarizing response induced by dim light spots decreased when the diameter of the spot was increased beyond about 1,000 micrometer. 3. With bright-light stimuli the peak amplitude of the light responses increased monotonically when increasing the illuminated area, but a delayed antagonistic effect appeared with spots of more than 1,000-micrometer diameter. 4. Depolarizing responses were observed if the periphery of the receptive field was stimulated with annuli of light in the presence of central background illumination. 5. The interaction of the inputs converging on the L2-HC from the peripheral and central regions of the receptive field was found to be nonlinear. The same peripheral stimulation could result either in an enhancement or in a depression of the central response according to the intensity of the central illumination. 6. By comparing the receptive-field properties of the L2-HC and the red cones a model is proposed that explains the antagonistic surround mechanism of the L2-HC on the basis of the antagonistic peripheral mechanism in cones. '