1. We used standard, single-cell recording techniques to study the response properties of 34 W-cells in the C-laminae of the cat's lateral geniculate nucleus. By W-cell, we mean a poorly responsive geniculate neuron that receives slowly conducting retinal afferents; these are quite distinct from geniculate X- and Y-cells. Our measurements included response latency to optic chiasm stimulation, plots of the receptive-field center, time course of response, and responses to counterphased, sine-wave gratings. This last measurement also involved the determination of contrast sensitivity, which is defined as the inverse of the contrast needed to evoke a threshold response at a particular spatial and temporal frequency of the grating. Many of these responses were compared to those of geniculate X- and Y-cells recorded in the A-laminae. 2. Each of the W-cells responded with a latency of at least 2.0 ms to optic chiasm stimulation, and most (76%) exhibited a latency of at least 2.5 ms. However, only 26 of these W-cells responded to visual stimuli, and these responses were weak or "sluggish," as has been reported previously. Receptive fields of these W-cells tended to be large, compared to those of X- and Y-cells, and included 11 on-center, 13 off-center, and 2 on-off center fields. 3. W-cells exhibited either linear (12 cells) or nonlinear (14 cells) spatial and temporal summation, as determined from their responses to counterphased, sine-wave gratings. Linearity of spatial summation was determined by measuring contrast sensitivity as a function of the grating's spatial phase. The linear W-cells' responses were sinusoidally phase dependent, and the nonlinear W-cells' responses were independent of spatial phase. Linearity of temporal summation was determined by the presence or absence of harmonic distortion in the response relative to the grating's counterphase rate. Linear W-cells responded chiefly at the grating's fundamental temporal frequency, whereas much of the nonlinear W-cells' responses occurred at the second harmonic of the grating's temporal frequency. Thus, nonlinear W-cells exhibited many of the characteristics previously described for Y-cells. 4. Spatial and temporal contrast-sensitivity functions were determined for seven linear and eight nonlinear W-cells. Overall sensitivity values of the linear and nonlinear W-cells were comparable, but these groups differed in terms of the nature of the response component (linear or nonlinear) that was more sensitive. 5. The linear W-cells in our sample included both tonic (comparable to the "sluggish-transient" type of retinal ganglion cells) types, while all nonlinear W-cells were phasic. Otherwise, no difference between linear and nonlinear W-cells was seen for latency to optic chiasm stimulation, receptive-field size, overall contrast sensitivity, responsiveness to visual stimuli, overall spatial resolution, or temporal resolution. 6...