Data from several experiments on long-term potentiation (LTP) in the rat hippocampus were examined for circadian influence. Incidence and magnitude of LTP produced in both area CA1 and area dentata were analyzed, and a reciprocal light/dark difference was found in the two areas, with pyramidal cells of area CA1 showing more LTP during the light period and granule cells of area dentata showing more LTP during the dark period. In addition, results from experiments on developing animals, suggested that the circadian influence on LTP in either area was not present before postnatal day 20. All of these experiments were from hippocampal slice preparations; therefore, it is important to note that circadian influences on hippocampal LTP are preserved in the in vitro environment where tonic extrahippocampal input has presumably been removed.