1. The ability of cats to distinguish colours was investigated at mesopic and photopic levels to test the hypothesis that cats discriminate wavelength by using rods in conjunction with a single type of cone.2. Cats were trained to distinguish red from cyan, and orange from cyan at the mesopic level. They retained the ability to make this discrimination when the coloured stimuli were placed against a background bright enough to saturate the rods.3. One cat was also tested after being exposed to a bright white light of 9000 cd/m(2) for a period of 5 min, and found able to distinguish red from cyan.4. These results suggest that cats have more than one type of cone. Subsequent recordings from single units in the lateral geniculate nucleus showed that there are rare opponent colour units in layer B with input from a green-absorbing cone and a blue-absorbing cone.