We have found that two distinct forms of long-term depression (LTD), one dependent on the activation of NMDA receptors (NMDARs) and the other dependent on the activation of metabotropic glutamate receptors (mGluRs), coexist in pyramidal cells of the CA1 region of the hippocampus of juvenile rats (11-35 days). Both forms were pathway specific, required membrane depolarization, and were blocked by chelating postsynaptic Ca2+ with BAPTA. The mGluR-LTD, but not the NMDAR-LTD, was blocked by the T-type Ca2+ channel blocker Ni2+ and intracellular administration of a protein kinase C inhibitory peptide. In contrast, the protein phosphatase inhibitor Microcystin LR blocked NMDAR-LTD, but not mGluR-LTD. NMDAR-LTD is associated with a decrease in the size of quantal excitatory postsynaptic currents, whereas for mGluR-LTD there was no change in quantal size, but a large decrease in the frequency of events. While mGluR-LTD did not interact with NMDAR-dependent long term potentiation (LTP), NMDAR-LTD was capable of reversing LTP. Prior saturation of mGluR-LTD had no effect on NMDAR-LTD. NMDAR-LTD and mGluR-LTD thus appear to be mechanistically distinct forms of synaptic plasticity in that they share neither induction nor expression mechanisms.