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 antagonizing baclofen-induced hyperpolarisations, mediated through gamma-aminobutyric acid (B) GABAB postsynaptic receptors 1. In electrically stimulated brain slices preloaded with [3H] GABA, CGP 36216 increased the release of [3H] GABA, which was reversed by baclofen. While CGP 36216 is ineffective at GABAB postsynaptic receptors, it is appreciably more active at presynaptic receptors1.