Abstract 1. 1. The free amino acids content in the central nervous system of the scorpion Centruroides limpidus limpidus, the crab Gecarcinus lateralis and the insect Lethocerus angustipes was studied. 2. 2. In the same tissues, the activity of glutamic-pyruvic (GPA), glutamic-oxalacetic (GOA) and γ-aminobutyric-α-ketoglutaric (ABAT) aminotransferases, as well as the glutamic acid decarboxylase (GAD) activity, were measured. 3. 3. The results obtained are compared to date from other species. Aspartic acid, glutamine, alanine, serine+glycine, lysine and particularly proline were found to be in considerably higher concentrations in the arthropods studied, especially in the crab, than in any other species reported. In contrast, γ-aminobutyric acid (GABA) levels are similar in arthropods and vertebrates. 4. 4. GPA was found to be as active as GOA in arthropods' nervous tissue, in contrast to the results previously obtained in mouse brain. ABAT activity was found higher than GAD activity in Gecarcinus, whereas the two activities are similar in mouse brain. 5. 5. The possible metabolic significance of the reported differences is discussed in relation to probable physiological roles of free amino acids in the nervous tissue of invertebrates.