Homosynaptic long-term depression (LTD) was studied in hippocampal slices from 12–18-d-old rats using field EPSP recording in the apical dendritic layer of CA1 pyramidal cells. Independent estimates of the alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and the N- methyl-D-aspartic acid (NMDA) receptor-mediated components of the field EPSP were obtained in parallel using early and late measurements of a dual-component EPSP in a low-magnesium solution. LTD was induced by low- frequency stimulation (LFS; 2 Hz for 10 min), resulting in equal relative changes of the AMPA and NMDA receptor-mediated components. Under conditions when the AMPA receptor-mediated component was fully blocked, a similarly sized LTD was observed for the pure NMDA receptor- mediated EPSP (measured as initial slope or peak amplitude). Equal changes in AMPA and NMDA receptor-mediated components occurred also upon application of the adenosine agonist N6-cyclohexyladenosine (CHA), known to act by decreasing transmitter release. On the other hand, LTD was found to interact in a multiplicative manner with the presynaptic release changes induced by CHA and by paired-pulse facilitation. The induction of the LTDs of both AMPA and NMDA receptor-mediated EPSPs was blocked by the NMDA receptor antagonist D(-)-2-amino-5- phosphonopentanoic acid and by the phosphatase inhibitor okadaic acid. Partial blockade of LTD by okadaic acid resulted in equal partial blockade of the LTDs of the AMPA and NMDA receptor-mediated components. On the other hand, the L-type calcium channel blocker nifedipine, the metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4- carboxyphenylglycine, the nitric oxide synthase inhibitor N omega-nitro- L-arginine, and the heme oxygenase inhibitor protoporphyrin IX zinc(II) had no effect on LTD of either the AMPA or the NMDA receptor-mediated component. These results of equal changes of AMPA and NMDA receptor- mediated components of the field EPSP in association with LTD, and the consistent parallelism of effects or noneffects on these components by various receptor antagonists and enzyme inhibitors, seem more easily explained by a presynaptic locus for LTD than by a postsynaptic one.