Summary Cellular events associated with degeneration of the projection of the olfactory bulb to the molecular layer of the piriform cortex of the mouse have been studied with rapid-Golgi and Fink-Heimer impregnations and with the electron microscope. Four classes of axon terminals: s-l, s-d, f-l, and f-d, are differentiated on the basis of whether the synaptic vesicles are spherical or flattened and whether the axoplasm is lightly or darkly stained. The majority of s-l terminals, the predominant class in sublamina Ia of the molecular layer, degenerate after bulb ablation. Degeneration of axon terminals is associated with dilation and, eventually, degeneration of segments of dendrites in Ia. Both s-l and s-d terminals contribute to a partial reconstitution of the neuropil of Ia during the weeks after bulb ablation.