Oseltamivir, an antiviral drug used for the treatment of influenza, contains the L-glutamic acid motif in its chemical structure. We focused on this structural characteristic of oseltamivir and examined the pharmacologic effects of the drug on the nervous system in invertebrate and vertebrate animal models. Injection of oseltamivir or L-glutamic acid into silkworm (Bombyx mori) larvae induced muscle relaxation. Oseltamivir and L-glutamic acid inhibited kainate-induced rapid muscle contraction, but neither drug affected insect cytokine paralytic peptide-induced slow muscle contraction. In the mammalian system, mice (Mus musculus) treated intracerebrally with oseltamivir developed convulsive seizures. Hydrolyzed oseltamivir, the active form containing a carboxylic acid, evoked epileptiform firing of hippocampal neurons in rat (Rattus norvegicus) organotypic hippocampal slice cultures. These results are the first to demonstrate that oseltamivir exerts pharmacologic effects on the nervous system in insects and mammals.