The inorganic polycationic dye ruthenium red (RuR) exerts several effects on the nervous system when added in physiological solutions, both in vivo and in vitro. Part of these effects, including the paralysis observed in mammals after the systemic administration of RuR, can be accounted for by the binding of RuR to nerve ending membranes, which results in inhibition of Ca2+ influx through voltage-sensitive calcium channels and the consequent inhibition of neurotransmitter release. On the other hand, the administration of RuR into the cerebrospinal fluid induces intense convulsive activity, and its microinjection into the substantia nigra reticulata or the hippocampus leads to various motor behavior alterations that can be related to hyperexcitability of the neurons of the injected region. In addition, RuR penetrates the neuronal somata present in the area injected and induces cell destruction, which has been interpreted as an excitotoxic action of the dye. The penetration and the toxicity of RuR were also observed in primary neuronal cultures but did not occur in pure glial cultures, suggesting a selective action on neurons. In the present article the in vitro and in vivo effects of RuR are reviewed and discussed in terms of the usefulness of the dye as an interesting tool to study calcium channels linked to transmitter release, neuronal death mechanisms and the function of neural pathways.