Due to the relative simplicity of their nervous system, invertebrate animals were widely used in the past decades for studying the processes of excitability at membrane level, as well as the mechanisms of neuronal events and interneuronal communication. Parallel with investigating basic questions of neurobiology, lower animals have also been the object of toxicological studies, because simple invertebrate preparations with well-known physiological, biochemical and pharmacological characteristics proved to be excellent models for testing the action of natural and synthetic compounds important to human pharmaceutical research as well as in searching suitable chemicals for pest control. In the last ten-fifteen years with the growing interest towards environmental protection, a new field was opened for the application of invertebrates, namely, testing and monitoring the presence and harmful effects of anthropogenic toxic substances. Invertebrates are used today both as passive and as active biomonitors to detect and evaluate the level of pollution in a given ecosystem, and to study the effects and mechanisms of action of pollutants. Invertebrate nervous systems are suitable objects in clarifying the mechanisms of action of toxic chemicals at various levels of the neural regulation. Toxic influences can be reflected in behavioural alterations, by the modification of the function of different organs as well as the neural regulation, presented by examples on mussels and snails. In case of neurotoxicity, the targets of action are the elements of the nervous system. Alterations can occur in the permeability (ion channels) of the neuronal membrane influencing excitability, potential generation and propagation of nerve impulse, in the transmitter system (synthesis, release, elimination and binding to the receptors), in the interneuronal and neuroeffector connections responsible for co-ordinated and adequate responses to the internal and external challenges. For the future, it can be predicted that neurotoxicological research with new compounds cannot be effective without using invertebrate preparations, since, due to animal protection and restrictions in animal experimentation, the permission of the use of vertebrates is much more limited in most of the developed countries, and this trend is certainly to be expanded.