The functional organization of the brain of Sepia has been investigated by electrical stimulation. As a result several new divisions of the brain have been made. The pedal ganglion has been shown to consist of four parts: (1) the anterior chromatophore lobes innervating the skin and muscles of the anterior part of the head and arms; (2) the anterior pedal lobe innervating the arms and tentacles; (3) the posterior pedal lobe innervating the funnel, collar and retractor muscles of the head; (4) the lateral pedal lobes innervating the muscles of the eyes and tissues of the orbits. The palliovisceral (or visceral) ganglion, apart from the magnocellular lobe demonstrated by Young (1939), is shown here to consist of (1) a central palliovisceral lobe innervating the mantle, funnel and viscera; (2) a pair of lobes innervating the muscles of the fins; (3) a pair of posterior chromatophore lobes innervating the muscles of the chromatophores and skin of the mantle, fin and back of the head; (4) a pair of vasomotor lobes. Because of these new divisions the three main groupings of the suboesophageal neural tissue are now referred to as the anterior, middle and posterior suboesophageal masses corresponding to the old brachial, pedal and palliovisceral divisions. The suboesophageal centres are classified as lower motor centres and intermediate motor centres, depending on the kind of response they give to electrical stimulation and their peripheral connexions. In the supraoesophageal lobes, higher motor centres and silent areas are recognized. The silent areas include the vertical, superior frontal, subvertical, precommissural and dorsal basal lobes. Of the higher motor centres the anterior basal lobe is primarily concerned with the positioning of the head, arms and eyes, particularly during movements involving changes in direction while swimming. Such manoeuvres are brought about by the anterior basal lobe control over the fins and position of the funnel. The posterior basal lobe is here shown to consist of six main divisions: (1) the subvertical lobe; (2) the dorsal basal lobes; (3) the precommissural lobe; (4) the medial basal lobe; (5) the lateral basal lobe; (6) the interbasal lobe. The medial, lateral and interbasal lobes are higher motor centres. The lateral and medial basal lobes control movements of the chromatophores and skin; the medial basal lobe controls swimming, breathing, fin movements and various visceral functions. The interbasal lobe controls the movements of the tentacles. The optic nerves and the optic lobes, at their periphery, are electrically inexcitable. Electrical stimulation of the centre of the optic lobes evokes all the responses that can be obtained from the other higher motor centres. The results are discussed in terms of Sanders & Young's (1940) physiological classification of the brain. A further category intermediate motor centre is recognized. Summary lists of the responses of each lobe are given on pages 516, 520, 525.