In the awake resting cat, the tonic activity of the parasympathetic ocular nerves (short ciliary nerves, SCN) consists of a background of small amplitude impulses and of superimposed large amplitude impulses of lower frequency. Preliminary observations suggested: (a) that these two types of impulses were involved in separate functions; and (b) that only the small amplitude impulses had a pupillo-constrictor activity. The present work was undertaken to test this hypothesis. 1. 1. The experiments were carried out on cats, with the spinal cord divided at C 1 or at T 2–T 4 and immobilized with Flaxedil. The variations of the two kinds of impulses have been compared during spontaneous or evoked changes in cortical activity and pupil diameter. A direct method of counting each category of potentials on magnified oscilloscopic records was used in one series of experiments. In another series, the large amplitude impulses were electively recorded, using a voltage discriminator, and their variations were compared to the simultaneous variations of the integrated total activity of the SCN. 2. 2. It has been observed that: (a) the small amplitude impulses are regularly inhibited during cortical arousal and the accompanying pupillo-dilatation. Conversely, the discharge frequency of these small potentials undergoes a considerable acceleration when the pupil closes at the onset of sleep. Thus these small amplitude spikes may safely be identified with the pupillo-constrictor tonic impulses; (b) the large amplitude impulses vary either in parallel with or in the opposite direction from the small amplitude impulses. They are inhibited during reticular stimulation, but a rebound of their discharges occurs during post-stimulatory arousal, white the small impulses are still inhibited. They are sometimes inhibited during sensory stimulation: more often, however, an intense and prolonged discharge of these impulses, synchronous with the discharge of the sympathetic pupillo-dilator fibres, is elicited by acoustic, olfactory or tactile stimuli. On the contrary, their activity is greatly reduced or abolished at the onset of sleep. Such behaviour is what may be expected from the accomodation fibres, according to the observations of Elul and Marchiafava (1964) on non-specific refraction changes of the cat's eye. 3. 3. These results are discussed in relation to histological data on the vegetative ocular innervation and also to the findings of Whitteridge (1937) who described two kinds of post-ganglionic fibres in the SCN of the cat. They lead to the following conclusions: (a) that the efferent ocular parasympathetic pathways mediating the pupillo-motor and the accomodative function of the Edinger-Westphal nucleus differ in the electrophysiological properties of their post-ganglionic fibre, and also probably in their intra-ganglionic connections, and (b) that the striking lack of parallelism frequently observed between the discharges of the pupillo-constrictor and accomodation impulses, at various levels of central activation, implies separate supra-nuclear controls of the two main functions of the ocular parasympathetic innervation, even in the absence of visual information.