1. The A-laminae of the cat lateral geniculate nucleus (LGN) contain two distinct groups of relay neurons: lagged and nonlagged cells. The groups differ in the pattern, timing, and amplitude of response to flashing spots. At spot onset, nonlagged cells discharge at short latency with an excitatory transient; in lagged cells this transient is supplanted by an inhibitory dip and a delayed latency to discharge. At spot offset, lagged cell discharge decays more slowly than in nonlagged cells. Here we have investigated the facilitatory influence of the brain stem reticular formation on the response properties of lagged X-cells (XL) and nonlagged X- and Y-cells (XN and YN). We were particularly interested in whether the inhibitory dip and sluggish response of lagged cells could be reversed during brain stem activation and the cells induced to respond like nonlagged cells. The peribrachial region (PB) of the pontine reticular formation was stimulated electrically with the use of 1,100-ms-long pulse trains that were paired with flashing spot stimuli. 2. Stimulation of PB led to an increase in the amplitude of visually evoked discharge in lagged and nonlagged cells. Compared with their response to spot stimulation alone, the average PB-evoked increase in mean discharge rate was greater than 50% in both groups. The mean discharge rate during PB plus spot stimulation was somewhat higher for XN-cells than for YN- and XL-cells, reflecting the relatively higher discharge rate among XN-cells during spot stimulation alone. 3. Two measures of response timing characterize lagged and nonlagged cells: latency to half-maximal discharge at spot onset (half rise) and latency to half-minimal discharge at spot offset (half fall). Among XN- and YN-cells, PB stimulation had no significant effect on these two latencies; among XL-cells, both latencies were reduced by 43 and 35%, respectively, on average. 4. During spot stimulation alone, all lagged cells were distinguishable from all nonlagged cells in having half-rise and half-fall latencies greater than 60 ms. Despite the reduction among XL-cells in these 2 latencies during PB stimulation, all but 2 of the 40 XL-cells maintained laggedlike latencies. The majority (95%) of XL-cells remained unambiguously lagged on these measures during brain stem stimulation. 5. During spot stimulation alone, 30 of 40 XL-cells tested displayed a prominent and often long-lasting inhibitory dip in discharge starting approximately 45 ms after spot onset. During PB stimulation only three cells lost the dip.(ABSTRACT TRUNCATED AT 400 WORDS)