The latencies and visual response properties of 202 X-cells in the A-laminae of the cat dorsal lateral geniculate nucleus (LGN) were examined to investigate the recent claim (Mastronarde, '85,'87a) that functionally different groups of X-cells reside there. Two groups of X-cells were found, which differed in their extracellularly recorded responses to spots of light flashed within their receptive fields. One group, constituting one-third of the sample, responded to spot onset with a profound and often long-lasting dip in discharge rate, such that cell discharge usually did not reach half maximum until greater than or equal to 100 msec after spot onset. About 70% of these cells also displayed a transient discharge at spot onset. These cells correspond to Mastronarde's lagged X-cells, and we similarly refer to them as XL-cells. The second group, constituting the remainder of the X-cell population, generally responded to spot onset with a short latency (less than or equal to 60 msec) brisk discharge, no detectable XL-type dip, and a rapid reduction in firing at spot offset. We refer to these neurons as nonlagged (XN) X-cells; this group probably encompasses all of Mastronarde's non-XL-cells. Despite some overlap, the XL- and XN-cells differed in numerous other features. Compared to XN-cells, XL-cells exhibited: 1) lower peak rates of discharge and more uniform firing during spot onset; 2) slightly longer latencies and markedly lower probabilities of discharge to optic chiasm stimulation; 3) consistently lower geniculocortical conduction velocities; and 4) markedly lower optimal temporal frequencies when tested with drifting sine wave gratings. No differences were found between the two cell groups in optimal spatial frequency, spatial resolution, or receptive field center size, and there were equal proportions of on- and off-center types of XL- and XN-cells. Analyses of one- and two-dimensional plots of the physiological measures indicate that XL- and XN-cells constitute a physiological continuum. However, the two groups occupy opposite sides of the continuum on many of the measures, with little overlap and with few (less than 5%) cells with intermediate properties. Therefore, XL-cells may be considered a distinct, readily identifiable group. These findings confirm and extend Mastronarde's ('87a) observations on functional differences among geniculate X-cells.(ABSTRACT TRUNCATED AT 400 WORDS)