We investigated the effects of rolipram, a selective cAMP phosphodiesterase (PDE) inhibitor, on late plastic events during functional CA1 plasticity in vitro in rat hippocampal slices. We present data showing that an early form of long-term potentiation (LTP) (early-LTP) that normally decays within 2-3 hr can be converted to a lasting LTP (late-LTP) if rolipram is applied during tetanization. This rolipram-reinforced LTP (RLTP) was NMDA receptor and protein synthesis dependent. cAMP formation in region CA1 during late-LTP requires dopaminergic receptor activity (Frey et al., 1989, 1990). Thus, we studied whether RLTP was influenced by inhibitors of the D₁/D₅ receptor. Application of the specific D₁/D₅ antagonist SCH23390 (0.1 μm) did not prevent RLTP, suggesting that the phosphodiesterase inhibitor acts downstream of the D₁/D₅ receptors. We also studied whether rolipram can interact with processes of synaptic tagging, because RLTP was also dependent on protein synthesis, similar to late-LTP. Inhibition of PDE and subsequent induction of RLTP in one synaptic population were able to transform early-LTP into late-LTP in a second, independent synaptic population of the same neurons. This supports our hypothesis that cAMP-dependent processes are directly involved in the synthesis of plasticity-related proteins.