Increasing anatomical evidence indicates that large retinal ganglion cells (M-cells) are preferentially damaged in primary open angle glaucoma (OAG), while the smaller ganglion cells (P-cells) are relatively spared. In 13 patients with defined OAG and modest visual field defects, we evaluated the responses to stimuli that are expected to involve primarily the function of the P-pathway and compared them with those of control subjects. The psychophysical contrast sensitivity (CS), the PERG and the VEPs were measured for red-green gratings of pure chromatic contrast, as well as yellow-black gratings of pure luminance contrast. As compared with controls, OAG patients had reduced CS for both luminance and chromatic contrast stimuli by about 6 dB. PERGs and VEPs to luminance stimuli were little affected, whereas those to chromatic stimuli were both reduced in amplitude and delayed. These results indicate that visual dysfunction in glaucoma is not selective for the M-pathway, and that responses to equiluminant colour-contrast stimuli may be of diagnostic value.