Operant differentiation was used to study black/white stimulus intensity discrimination (over the range 1-37 cd/m2; CIE-31 coordinates X = 0.340, Y = 0.304) in two monkeys (rhesus macaques). Confusion matrices were constructed from the probabilities that animals would make operant responses when required to select from pairs (a conditioned stimulus and one of 9 differentiation stimuli) of stimuli in 10 series of experiments with conditioned stimuli of 10 intensities. Correlation matrices for vectors consisting of stimuli in the confusion matrices were assessed by factor analysis for identification of their intrinsic vectors. A perceptive space of two-dimensional structure was obtained for the brightness of black/white stimuli, and the main characteristics of this were similar to those of the analogous perceptive spaces for humans, fish (carp), and rabbits. The coordinate axes of this space can be interpreted in terms of excitation of two intensity-encoding channels: light and dark.