The possible enhancement of inhibitory inputs to relay cells of the dorsal lateral geniculate nucleus (dLGN) by serotonin (5-HT) was studied during blockade of GABAA-receptor mediated inhibition with bicuculline (BICU). Visually induced spike activity of single units was recorded during simultaneous micro-iontophoretic application of BICU or 5-HT. All 28 relay cells studied were disinhibited by BICU and strongly inhibited by 5-HT. The responses during BICU or 5-HT were on average statistically different from the controls (p < 0.01, Student's t-test). The inhibition by 5-HT could be almost totally abolished by the simultaneous application of BICU and responses during the combined application of both substances were only slightly different from those obtained during BICU alone (p < 0.3).