GABA B receptors form the basis of a powerful and versatile inhibitory system in the mammalian brain. Presynaptic and postsynaptic actions of GABA B receptors have been described in various brain regions, including the hippocampus. We report here on a novel pharmacological agent, presumably a peptide, which inhibits synaptic transmission in the CA1 area of the rat hippocampus via GABA B receptors. The agent is a component of a nootropic drug, Cerebrolysin T M , obtained from pig's brain extract. In contrast to other, presently known agonists, such as baclofen or GABA, Cerebrolysin T M acts preferentially on presynaptic GABA B receptors and has no detectable postsynaptic inhibitory effects. Additional, postsynaptic depolarizing action of the drug resulting in increased excitability is pharmacologically distinct from the GABA B response and partially masked by the inhibition. The presynaptic GABA B agonist may add to clinical effects of Cerebrolysin T M in treatment of brain injuries. Moreover, it promises to be a useful experimental agent in further studies of many possible functional roles of GABA B receptors.