The notion that blockade of constitutive protein synthesis underlies the effect of protein synthesis inhibitors (PSIs) on long-term potentiation (LTP) stabilization was examined using the rat hippocampal CA3-CA1 synapse. Using a biochemical assay we found protein synthesis rate largely recovered 1h after wash-out of cycloheximide (CHX). Nonetheless, a 4-h CHX application followed by wash-out 1h prior to LTP resulted in a significant decrement of LTP stabilization. Wash-out initiated just prior to LTP, thus extending protein synthesis inhibition well into the post-LTP period, resulted in no further effect on LTP. However, short pre- and continuous post-tetanization application of PSIs failed to influence LTP persistence for up to 7 h. Addition of hydrogen peroxide (H₂O₂) 5-25 min following LTP induction resulted in parallel depression of potentiated and non-potentiated inputs, leaving LTP seemingly unaltered. However, in the presence of cyxloheximide the H₂O₂ application resulted in a significant reduction of LTP.LTP stabilization was impaired by pre-LTP application of protein synthesis inhibition but not by post-LTP application unless the slices were exposed to oxidative stress. We submit that these results favor the notion that constitutive rather than triggered protein synthesis is important for LTP stabilization.