The presence of opioid, b-adrenergic and cholinergic receptors has been demonstrated in ciliated protozoa, but little is known about c-aminobutyric acid (GABA) receptors. In this study we have analyzed the distribution of GABAA-type receptor subunits in Paramecium. Confocal laser microscopy using antibodies specific for a1-, a2-, a3-, a6-, b2/3-, c2-, e-, k-, and h-subunits showed that most receptors are aggregated in clusters and are distributed both on cell surface and in the cytoplasm. The intensity of labelling of the a6-, b2/3- and c2-subunits was more intense than the a1-, e-, and h-subunits, suggesting that the former are present in higher concentrations than the latter. � 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.