Changes in the strength of synapses in the hippocampus that occur with long-term potentiation (LTP) or long-term depression (LTD) are thought to underlie the cellular basis of learning and memory. Memory formation is known to be regulated by spacing intervals between training episodes. Using paired whole-cell recordings to record from synapses connecting two CA3 pyramidal neurons, we now show that stimulation frequency and spacing between LTP and LTD induction protocols alter the expression of synaptic plasticity. These effects were found to be dependent on protein phosphatase 1 (PP1), an essential protein serine/threonine phosphatase involved in synaptic plasticity, learning and memory. We also show for the first time that PP1 not only regulates the expression of synaptic plasticity, but also has the ability to depress synaptic transmission at basal activity levels. Moreover, PP1 can sort two consecutive messages received by the postsynaptic neuron and control the direction of change in synaptic strength. This study highlights new roles of PP1 in regulating timing-dependent constraints on the expression of synaptic plasticity that may correlate with memory processes, and together PP1 and the spacing of stimulation protocols provide mechanisms to regulate the expression of synaptic plasticity at CNS synapses.