Heterosynaptic long-term depression (hetLTD) at one input can be induced by applying a conditioning stimulus to an adjacent set of synapses. In hippocampal CA1 pyramidal cells, our results suggest that hetLTD is triggered by an extracellular diffusible factor that is released following tetanic activation of NMDA receptors. This hetLTD occludes with homosynaptic LTD suggesting common underlying mechanisms.