Monkeys with lesions of cortical visual area V4 were compared with unoperated monkeys in three experiments. In Expt. 1 they were tested for the reacquisition of a pre-operatively learned hue discrimination task. In Expt. 2, as a test of colour constancy, the monkeys were required to perform previously overlearned colour discrimination tasks when the amounts of red, green and blue light in the illuminant were changed. In Expt. 3 the animals were compared on the post-operative acquisition of hue, greyness and saturation discrimination tasks. The results of Expt. 1 showed that monkeys with V4 lesions can regain their pre-operative levels of performance on hue discrimination tasks. Expt. 2 showed that monkeys with V4 lesions have a colour constancy deficit. Expt. 3 reinforced the finding that the animals with V4 lesions could learn to perform fine hue discriminations but that their final, asymptotic performance was not as reliable as that of normal animals. The wavelength discrimination data are discussed within the context of recent electrophysiological findings that V4 is involved in selective attention to visual stimuli and the constancy data are interpreted as evidence that V4 is important for defining colour constancy thresholds but not for constructing the perceptual categories underlying constant colour perception.