An octopus that has attacked a crab shown with a square and received a shock rapidly learns not to attack when this situation appears again, while continuing to attack crabs shown alone. The memory preventing attack on crabs shown with a white square may last for 2 or 3 days if the crab and square are not shown during that period. If the situation is shown three times a day the memory may last for 6 days or longer. The memory is not erased by anaesthesia nor by electrical stimulation of the supra-oesophageal lobes. After complete removal of the vertical lobe, or of the medial superior frontal lobe, or section of the tract between the two, the memory preventing attack is lost and cannot again be acquired. Animals operated in this way attack a crab and square if shown at 2-hourly intervals in spite of the numerous shocks they receive. A transitory memory lasting a few minutes can still be set up if the frequency of presentation is increased to about once every 5 min. Partial removal of the vertical lobe system does not interrupt the memory. A memory set up by the use of one eye is not abolished if the optic lobe of that side is later removed. The memory is not interrupted by slashes in both optic lobes. After lesions to the lateral parts of the superior frontal lobes an octopus makes few or no further attacks on crabs, unless these are placed close to the animal. The effect of such an operation is to upset the balance of central neural activities in such a way that a region responsible for inhibiting attacks on distant objects assumes control. This inhibitory region may be the first subvertical lobe, whose action is normally balanced by the lateral superior frontal lobes and the vertical lobe. The tangle of fibre bundles within the optic lobes allows for a wide degree of interaction between impulses arriving from different parts of the retinal surface. In addition, these lobes receive afferent fibres from the arms. They thus provide a system within which associations between given sets of inputs can be set up in such a way as to ensure that there is no attack when a similar set of inputs occurs again. Further plexiform arrangements are found in the pathway from the optic to the superior frontal lobes and from the latter to the vertical lobe. These plexuses make possible the interaction in each succeeding lobe of impulses arriving from distant parts of the preceding lobe. Each lobe can thus serve to record the pattern of associations present in the previous one. Since the arrangement is circular the pattern originating in the optic lobe is then re-presented back to it. It is suggested that the vertical lobe system serves to prolong memories set up in the optic lobes by re-presenting them from within, and thus allowing them to persist for long enough to produce some change of a more permanent nature.