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To date if has been demonstrated that an experimental animal, the sea slug Pleurobranchaea, is capable of one-trial food-aversion learning, and that the muscarinic antagonist scopolamine in low doses causes an enhancement of learning. Pharmacologic binding studies using a new, 125 sub I-form of quinuclidinyl benzilate, in addition to studies using the 3 sub H-form of this ligand, have uncovered not only the classical types of muscarinic receptors that are typical of vertebrate cortex, but also a new form that is not found in other invertebrates that we have tested. Usually muscarinic receptors are found in low densities in invertebrate neural membranes, but the density of the new form in this animal's neural membranes is similar to the density of the classic receptors in mammalian cortex. Neurophysiological studies of individual neurons in small groups of identifiable neurons have shown that their activity is variable, as is the behavior that they take part in generating, and that the variability fits the definition of low-dimensional chaos. Findings show that such variability is an important feature of the emergence of adaptive responses arising from parallel, distributed neural networks in biological systems.