The gamma-aminobutyric acid (GABA) is the major and the best known neurotransmitter with inhibitory properties in the central nervous system (CNS). Outside the CNS, GABA acts as a regulator of muscle tension and controls the secretion of hormones. In the peripheral region, as in the CNS GABA's function is known to be mediated by GABAA and GABAB receptors. Several recent reports have suggested a relationship between the GABA-ergic system and oncogenesis. It has been confirmed that both GABA content and GAD activity are increased in material from colon, breast, digestive tract, and ovarian cancer. In the light of theory of dynamic balance between stimulating and inhibitory amino acids, disturbances in GABA metabolism may be a sign of the cell's defensive reaction during carcinogenesis.