Pupillary responses to stimuli which favour the preferential stimulation of neural mechanisms involved in the detection of visual attributes such as colour, spatial structure, movement and light flux changes on the retina have been measured and compared. Pupil responses to a decrement in stimulus luminance (i.e., a flash of darkness), suggest that at least three components are involved in this response, their relative contribution being determined largely by stimulus size, contrast and presentation time. A comparison of pupil responses to gratings of equal and lower space-averaged luminance shows that the amplitude of pupillary constriction at grating onset for the equal luminance condition is about twice that measured with similar gratings in the lower luminance condition. Pupillary responses to chromatic isoluminant gratings are in general of longer latency when compared to responses of similar amplitude elicited by achromatic gratings. Small pupillary constrictions elicited by the onset of coherent movement in dynamic, random dot patterns are also demonstrated under stimulus conditions which eliminate pupillary responses to sudden light flux changes on the retina. The results support an earlier hypothesis which suggests that the onset of sudden changes in neural activity in the visual cortex when a visual stimulus is presented to the eye causes an overall perturbation which weakens transiently the regulatory inhibitory input to the pupillomotor nucleus. This, in turn, results in a transient increase in the efferent parasympathetic innervation of the iris sphincter muscle and hence the observed constriction of the pupil. The characteristics of the pupillary response reflect the properties of the mechanisms and the number of neurones which participate in the detection of each stimulus attribute.