1. In a perfused eyecup preparation, AII amacrine cells of the cat retina were penetrated with glass microelectrodes and their electrical responses to photic stimuli recorded. 2. Intracellular injections of the stains Procion, lucifer, or horseradish peroxidase revealed dendritic tree diameters of 30-80 micrometers (48 +/- 16 micrometers, mean +/0 SD) and cell body diameters from 7 to 12 micrometers (9 +/- 3 micrometers) for these cells. The dendrites were broadly stratified throughout the inner plexiform layer (IPL) but possessed large, terminal varicosities in the IPL and inner nuclear lyer (INL) proximal to the cell body. 3. The waveform of these cells in response to photic stimulation suggested division into four components: a) an initial rapid depolarization of the cell membrane followed by a slower decay toward the dark level; b) suppression of the dark noise of the cell; c) with dim or moderately intense stimuli, an off-hyperpolarization; d) in some cases a hyperpolarizing surround response. 4. The receptive fields of AII cells have been characterized using spatial stimuli consisting of long narrow slits. Curves have been fitted to spatial data using two space constants, one for the center mechanism and an opposing one for the surround. For the central mechanism, space constants ranged from 20 to 80 micrometers (46 +/- 22 micrometers), while for the surround they ranged from 60 to 130 micrometers (85 +/- 28 micrometers). The mean half-width of the center mechanism, calculated from the mean space constant, was about 0.25 degrees of visual angle (64 micrometers). The receptive-field properties of AII amacrine cells resemble those of center-depolarizing bipolar cells of other species. 5. Spectral studies of AII amacrine cells reveal that they are rod driven at all criterion voltage levels. Furthermore, adaptation of the rods by rod-saturating backgrounds eliminates 95% of the response amplitude of the AII amacrine cells. Under these conditions the tiny response component remaining is driven by the cat's long wavelength (556-nm peak) cones. 6. AII amacrine cells depolarize to rod stimulation more rapidly than other rod-dominated cells, such as rod bipolar cells, which hyperpolarize. For stimuli corresponding to about 10% of rod saturation, the latency to half-maximum amplitude is about 65 ms for AII cells, 40 ms faster than rod-dominated hyperpolarizing units. The leading edge of the response waveform for AII cells is also much more restricted in time. With the above stimulus it requires about 20 ms to increase from 25 to 75% of its peak, a period almost 4 times shorter than required by rod-dominated S-potential responses. With saturating stimuli the AII response requires only 5 ms to increase from 25 ot 75% of its peak. 7. Although prominent in the rod system, AII amacrine cells do not appear to be able to detect single quantum events. Threshold signals require the bleaching of about 200 rhodopsin molecules within a receptive field containing some 1,300 rods...