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Octopuses are high organized invertebrates with big brains and complex behavior. The architecture of their brain is very different than in mammals or other vertebrates. However, they have a high level of behavioral plasticity, which, obviously, has the neuronal substrate. Understanding the functioning of the octopus brain will expand our knowledge of the mechanisms of neuronal control of behavior. For this, we need the instruments, and the registration of local field potentials (LFP, or less precise term electroencephalogram) is one of the most simple, cheap, but powerful methods. Recording LFP in nonanesthetized freely-behaving octopus is challenging. Using wired devices is problematic because of the number, length, and strength of animals' arms. Wireless radio-based systems are not suitable because of the shielding properties of water which is a mandatory component of the cephalopods' environment. To solve this problem, we have developed a specialized water-resistant container for the NeuroLogger® (Newbehavior AG, Zurich, Switzerland) device. The electrodes (4 data and 1 reference) were implanted into the brain of three adult Octopus cyanea (2.5-4.2kg) under anesthesia. The capacity of the device was enough to obtain approximately 13 hours of LFP records while the animals were in the housing tank. The animals were filmed, then the video was synchronized to LFP, and behavior was manually scored by two experts. Unlike some previous reports, the LFP from octopus brains did not look like that of mammalian brains with regular oscillations during at least some behavioral states. LFP waves were mostly irregular and slow (<1Hz), with short periods of sinusoid-like activity or spikes. To reveal the possible correlation between LFP and behavioral states, we split the record into 10 s epochs and calculated the root mean square (RMS) of the signals' amplitudes. Three distinct behavioral states were selected for this analysis: sleeping (SL), non-sleeping not moving (NM), and non-sleeping actively moving (MV). Then RMS values were compared between different states for each animal. There were significant differences between at least two states in every channel and all animals. Interestingly, in some electrodes sleep activity had greater amplitudes than awake (both active and quiet). In others, the difference was the opposite. Since there is no stereotaxic atlas for the octopus brain, the positions of electrodes could be slightly different, and this results in some inconsistency across the animals. We need to collect further data to reveal the electrophysiological correlates of the octopuses' behavioral patterns. This may be real using our approach to record the brain electrical activity of these animals. [ABSTRACT FROM AUTHOR]