Long-term potentiation (LTP) with its extremely long duration has been frequently regarded as an elementary mechanism of information storage in the nervous system or at least as a suitable model for the study of mechanisms underlying functional plasticity and processes of learning and memory formation. Considering the necessity of an increased protein synthesis for memory consolidation and for the maintenance of LTP in granular synapses in vivo it was of interest to determine whether the LTP of the CA1 region of the hippocampus depends on protein synthesis as well. For the solution of this question anisomycin (ANI), a reversible blocker of protein synthesis, was used at a concentration of 20 μM, which blocked the [³H]leucine incorporation in hippocampal slices by at least 85%. It has been shown that in the CA1 region in vitro the maintenance of LTP (i.e. a late phase >5 h) depends on an ongoing protein synthesis. A 3-h treatment with ANI immediately following multiple tetanization resulted in gradually developing loss of field excitatory postsynaptic potential (EPSP) and population spike (PS) potentiation (15 ± 19% increase of the PS instead of the 96 ± 14% increase in non-treated control experiments at the 8th h after tetanization). Furthermore, a late PS potentiation (>6 h) of a second non-tetanized pathway to CA1 pyramidal cells has been observed (increase by 64 ± 18% at the 8th h) for the first time. This potentiation was ANI-sensitive as well and suggests that the maintenance of LTP is dependent on a postsynaptic mechanism.