An electrophysiological study has been made of the degeneration of the afferent axons of the lateral olfactory tract (L.O.T.) which give rise to excitatory synapses throughout the olfactory cortex. These fibres were severed by removing one or both olfactory bulbs. Evoked potentials were recorded from slices of olfactory cortex in vitro prepared at various times after bulbectomy. Up to 1.6 days later, all potentials were similar to those of the unoperated side. Between 1.6 and 1.8 days there was a complete loss of evoked potential from both L.O.T. axons and postsynaptic neurones, which occurred synchronously at all points along a 5-6 mm length of the tract.