CGP 36216 hydrochloride is a potent and selective antagonist of GABAB receptors with IC50 value of 43 µM. GABAB receptors (GABABR) are metabotropic transmembrane receptors for gamma-aminobutyric acid (GABA) and are linked through G-proteins to potassium channels. Expression of GABAB receptors are found in the central as well as in the autonomic division of the peripheral nervous system. GABAB receptors play a key role in regulating membrane excitability and synaptic transmission in the brain. In rat neocortical preparations maintained in Mg2+-free Krebs medium, CGP 36216 acts as antagonism of baclofen-induced suppression of spontaneous discharges in a concentration-dependent manner. However, CGP 36216, up to 1 mM, was ineffective in antagonising baclofen-induced hyperpolarisations, mediated through gamma-aminobutyric acid (B) GABAB postsynaptic receptors . In electrically stimulated brain slices preloaded with [3H] GABA, CGP 36216 increased the release of [3H] GABA, that was reversed by baclofen. While CGP 36216 is ineffective at GABAB postsynaptic receptors, it is appreciably more active at presynaptic receptors.