1. The graded, synaptic potentials of first-order visual interneurons (lamina monopolar cells) were examined with intracellular recordings. The spatiotemporal properties were characterized with drifting sine wave gratings and annuli. 2. Annulus-elicited inhibition is maximal for annulus-test pulse intervals of approximately 140 ms and declines exponentially. The inhibition declines with increasing annular internal radii (ri). 3. Grating responses were examined with respect to spatial and temporal frequency. The gratings elicit sinusoidal signals that are approximately linear with contrast. 4. Variations in spatial frequency produce response functions with a low-pass or modest band-pass characteristic, which are described by a difference of Gaussians sensitivity profile. The central Gaussian approximates the sensitivity profile of photoreceptors. The inhibitory Gaussian is similar to the inhibitory field estimated with annulus pulses. The peak of the inhibitory Gaussian is approximately 18% of the peak excitatory Gaussian. 5. Variations in temporal frequency generally produce transfer functions with a band-pass characteristic and a peak at 1.0 Hz. These data were described by a difference of exponentials function convolved with a low-pass filter that approximates the photoreceptor response. The inhibitory time course estimated from these data was similar to that of the annulus measurements. 6. The spatiotemporal properties of lateral inhibition are consistent with inhibitory action by the lamina amacrine neurons. The proposed model is spatiotemporally inseparable and nonrecurrent. 7. Eleven of 20 monopolar cells tested exhibited a strong orientation preference with a bias to the vertical. Photoreceptors exhibit little or no orientation preference.