[3H]2-deoxyglucose autoradiography has revealed foci in the antennal lobe of Drosophila melanogaster that are active during olfactory stimulation. Labelling in the receptor axons and the lobe interneurons can be observed. Each class of odors stimulates activity in a specific subset of antennal glomeruli. This defines the activity domain, which is recognizably distinct, although overlapping, for different classes of volatile chemicals. Unilateral stimulation with attractants predominantly results in excitation of the ipsilateral glomeruli. Collaterals of the receptor neurons to the contralateral lobe stimulate no visible postsynaptic activity. On the other hand, unilateral stimulation with repellent odors, such as benzaldehyde, results in an equally strong activity in both antennal lobes.