Until the last few decades, it was believed that only one optical isomer of amino acids occurred naturally; however, many D-amino acids were reported to occur in plants and bacteria, but their function has been unknown until recently. In the last 25 years some D-amino acids, in particular D-aspartate, have been discovered to occur in animal tissues, above all in the endocrine and nervous system. Free D-Asp has been found at high concentration in central and peripheral nervous system and in the reproductive tissue of the cephalopods Octopus vulgaris, Loligo vulgaris, and Sepia officinalis in both adults and embryos. In mammals (human and rat) this enantiomer occurs in central nervous system and in the endocrine glands, above all, in the adenohypophysis and pineal glands, where it is involved in hormone release and synthesis. There is evidence that in the nervous system D-Asp enhances the N-methyl-D-aspartate receptors, and the reduction of D-Asp in human brain has been correlated with some neuropathology, such as Alzheimer`s disease. D-Asp is present in the gametes and in embryos, where it plays a role during the first stage of development. D-Asp appears to have biphasic activity. When it is given at consistent doses to animals, it provokes damage and induces inhibition of growth. On the contrary, at the physiological concentration (nmol levels) in animals, it possesses a neuronal and hormonal role.