AbstractThe amino acid gamma‐aminobutyric acid (GABA) is an important inhibitory neurotransmitter in both vertebrates and invertebrates. Despite this, no reports of GABA in flatworms have to date been published. We have studied the presence of GABA in the planarian Dugesia tigrina with immunocytochemical methods and high‐pressure liquid chromatography. Fibers showing GABA‐like immunoreactivity (GABA‐IR) were present in abundance in the longitudinal nerve cords and lateral nerves. GABA‐IR was revealed in fibers forming commissures in the brain. The ventral part of the subepidermal plexus showed GABA‐IR. No cell somata containing GABA‐IR could be identified with certainty.The chromatographic analysis showed that the average GABA concentration in D. tigrina is 533.6 pmol/mg protein. This is substantially higher than the concentrations of dopamine (62.87 pmol/mg) and serotonin (233.20 pmol/mg). An enzyme assay confirmed the capacity for GABA‐synthesis in D. tigrina. The results indicate that GABA‐containing neurons appeared earlier in evolution than was previously thought and that GABA may serve an important role already in the flatworms. © 1994 Wiley‐Liss, Inc.