Fluids were collected from the rabbit neocortex during a tetanic stimulation of the cortical surface. When these samples from the neocortex were applied on the guinea-pig hippocampal slices, only those fractions containing substances with molecular weights less than 3000, 3000-10,000 and greater than 50,000 and not with other molecular weights, could induce long-term potentiation of population spikes in the CAI area in response to stratum radiatum stimulation. Intracellular recordings from the CAI neurons revealed that the long-term potentiation-inducing substances increased the excitatory postsynaptic potential without changing the membrane potential and the input resistance of these cells. A pretreatment of the rabbits with MK-801 prevented the release of the long-term potentiation-inducing substances. 2-Amino-5-phosphonovalerate was unable to block the long-term potentiation-inducing action of the substances from the rabbit neocortex. Gel-electrophoresis of the substances collected from the rabbit neocortex revealed the presence of an acidic peptide with a molecular weight of about 69,000. These results indicate that tetanic stimulation of rabbit neocortex results in a release of substances with molecular weights of less than 3000, 3000-10,000 and greater than 50,000 that could induce long-term potentiation in guinea-pig hippocampal slices. The release, but not the long-term potentiation-inducing action, of these substances appears to depend on the activation of N-methyl-D-aspartate receptors. The long-term potentiation-inducing substance in the greater than 50,000 mol. wt fraction may be an acidic peptide with a molecular weight of about 69,000.