In an immunocytochemical study, using an antiserum and a monoclonal antibody specific for the amino acid, gamma-aminobutyric acid (GABA), GABA-like immunoreactivity (GLIR) has been demonstrated for the first time in parasitic flatworms. In Moniezia expansa (Cestoda), GLIR was seen in nerve nets which were closely associated with the body wall musculature and in the longitudinal nerve cords. In the liver fluke Fasciola hepatica (Trematoda), the GLIR occurred in the longitudinal nerve cords and lateral nerves in the posterior half of the worm. GLIR was also detected in subtegumental fibres in F. hepatica. The presence of GABA was verified, using high-pressure liquid chromatography coupled with fluorescence detection. The concentration of GABA (mean +/- S.D.) in M. expansa anterior region was 124.8 +/- 15.3 picomole/mg wet weight, while in F. hepatica it was 16.8 +/- 4.9 picomole/mg. Since several insecticides and antinematodal drugs are thought to interfere with GABA-receptors, the findings indicate that GABAergic neurotransmission may be a potential target for chemotherapy in flatworms too.