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1. Mesopic increment threshold spectral sensitivities of sixty-six on-centre and twenty-five off-centre ganglion cells in the cat were determined by recording from single fibres of the left optic tract at a level posterior to the optic chiasma.

2. All units were monocularly driven; receptive fields were located almost exclusively in the right visual half-fields within 30° of the area centralis, but with slight overlap across each retinal mid line to the left half-fields. The extent of field spread to the right temporal hemi-retina was significantly larger than that to the left nasal hemi-retina. Field centre diameters ranged from less than 0·25° for central units to 2° for more peripheral units.

3. At high mesopic adaptation of 1 log cd/m² all responsive units (forty-four fibres) received mixed cone—rod input. Threshold curves could always be fitted by the absorption spectra of visual pigment 556 together with varying contributions from visual pigment 507, each derived from the Dartnall nomogram.

4. Of forty-seven fibres analysed under low mesopic background (0 log cd/m²) 92% received similar cone—rod input, being fitted predominantly by visual pigment 507 with slight cone contamination. The remaining 8% received pure rod input and could be matched by visual pigment 507 alone.

5. In conclusion, the cat retina presumably contains a single class of cones with absorption maxima at 556 nm, and a single class of rods. Discrepancy between the presumed rod absorption maximum (502 nm) and the low-mesopic sensitivity maxima of tract fibres (507 nm) is considered in terms of tapetal reflectivity, and absorption by ocular media. Both mechanisms input to the great majority of retinal ganglion cells. At high mesopic levels the cone mechanism predominates. At low mesopic levels the rod mechanism predominates. A small proportion of ganglion cells within the central 30° of the retina receive input only from rods, and in these the rod mechanism saturates completely below 1 log cd/m².