Guinea pigs were unilaterally bulbectomised and the contents of aspartate, glutamate and GABA measured in slices of olfactory cortex taken from the lesioned and intact hemispheres. Two days after the operation there was a fall in the aspartate and glutamate levels, which persisted for over 120 days, whereas gamma-aminobutyric acid (GABA) showed a transient fall followed by a small rise. The fall in glutamate and aspartate was much greater in small, thin slices containing a high density of nerve terminals. The synthesis of 13C aminoacids from [13C]glucose during electrical stimulation was greater in the slices taken from the normal side than in those from the operated side. The GABA synthesis, however, was four times greater on the lesioned side. This time-course for the fall in acidic amino acids correlates with the fall in electrical responses, and this lends weight to the idea that aspartate and/or glutamate mediate synaptic transmission in the area.