Surgical, pharmacological and genetic lesion studies have revealed distinct anatomical sites involved with different forms of learning. Studies of patients with localized brain damage and work in rodent model systems, for example, have shown that the hippocampal formation participates in acquisition of declarative tasks but is not the site of their long-term storage. Such lesions are usually irreversible, however, which has limited their use for dissecting the temporal processes of acquisition, storage and retrieval of memories. Studies in bees and flies have similarly revealed a distinct anatomical region of the insect brain, the mushroom body, that is involved specifically in olfactory associative learning. We have used a temperature-sensitive dynamin transgene, which disrupts synaptic transmission reversibly and on the time-scale of minutes, to investigate the temporal requirements for ongoing neural activity during memory formation. Here we show that synaptic transmission from mushroom body neurons is required during memory retrieval but not during acquisition or storage. We propose that the hebbian processes underlying olfactory associative learning reside in mushroom body dendrites or upstream of the mushroom body and that the resulting alterations in synaptic strength modulate mushroom body output during memory retrieval.