Parallel fiber synapses onto Purkinje neurons in acute cerebellar slices undergo long-term depression (LTD) when presynaptic activity coincides with postsynaptic depolarization. These electrical inputs can be respectively replaced by nitric oxide (NO) and Ca2+ photolytically released inside the Purkinje neuron, showing that these two messengers are sufficient for LTD induction. NO acts via cGMP production because inhibitors of guanylate cyclase prevent LTD but can be circumvented by photoreleased cGMP combined with Ca2+ elevation. Three inhibitors of cGMP-dependent protein kinase, Rp-8Br-PET-cGMPS, KT5823, and a novel pseudosubstrate peptide, all block LTD. LTD induction permits <10 ms gap between NO release and Ca2+ elevation, whereas 200-300 ms is allowed between uncaged cGMP and Ca2+ increase. This surprising difference in timing precision can be explained either by tighter localization and faster decay of cGMP when generated by NO rather than uncaging, or by two independent coincidence detectors in series.