The pattern of impulse transfer along the entorhinal-hippocampal-entorhinal loop has been analyzed in the guinea pig by field potential analysis. The loop was driven by impulse volleys conducted by presubicular commissural fibers, directly stimulated in the dorsal psalterium, which monosynaptically activated perforant path neurons in the medial entorhinal cortex. Perforant path volleys activated in sequence the dentate gyrus, field CA3, field CA1, subiculum, and entorhinal cortex. Input-output curves were reconstructed from responses simultaneously recorded from different stations along the loop. The entorhinal response to the presubicular volley was found to increase gradually with respect to its input. The population excitatory postsynaptic potential (EPSP) of the dentate gyrus granule cells had a similar behavior. By contrast, the input-output relation between the granule cell population spike and population EPSP was described by a very sleep sigmoid curve. The population spike of CA3 and CA1 pyramidal neurons as well as the response evoked in the entorhinal cortex by the hippocampal output had slightly higher threshold than the granule cell population spike and, like the latter, abruptly reached maximum amplitude. These findings show that the entorhinal-hippocampal-entorhinal loop transforms a linear input in a non-linear, almost all-or-none output and that the dentate gyrus is the critical site where the transformation occurs. Beyond the dentate gyrus, the loop appears very permeant to impulse traffic.