Abstract A rise in Ca 2+ concentration at postsynaptic sites provides an initial step in inducing both the long-term potentiation (LTP) and long-term depression (LTD) in the CA1 region of the hippocampus. LTP induction requires the activation of Ca 2+-sensitive protein kinases following the rise in Ca 2+. By contrast, the activity of protein phosphatase(s) appears to be critical to induce LTD. Here we demonstrate that inhibition of the synthesis of calcineurin Aα and Aβ, catalytic subunits of Ca 2+/calmodulin-(CaM) dependent protein phosphatase, reduces the threshold of induction for commissural-CA1 LTP in anesthetized rats. In rats administered antisense oligodeoxynucleotides (ODNs) against calcineurin Aα and Aβ intraventricularly for 7 days, a brief tetanic stimulation to the CA3 region, which in the control case was below threshold for the induction of LTP, now produced a long-lasting increase in both the EPSP slope and the amplitude of population spike recorded from the commissural-CA1 pathway. Western blot analysis of calcineurin showed that the threshold reduction was accompanied by a selective decrease in the protein levels in the hippocampus. Thus our study provides direct evidence that calcineurin per se has an antagonizing role in LTP induction. Complementary experiments with the selective calcineurin inhibitor, FK506, also showed the reduction of LTP threshold in a dose-dependent manner. These results, together with previous studies, support the hypothesis that the quantitative phosphorylation level of critical intracellular proteins determines whether the synaptic efficacy will increase or decrease after the activity-dependent rise in postsynaptic Ca 2+.