Each antennal lobe in the brain of a male moth has a distinctive neuropil structure, the macroglomerular complex (MGC), which is specialized for primary processing of information about the conspecific female sex-pheromone blend. Olfactory interneurons with dendritic arborizations in the MGC were examined by means of tandem intracellular recording and staining with Lucifer Yellow. Neurons that responded selectively to stimulation of the antenna with the major pheromone component, (E,Z)-10,12-hexadecadienal, had arborizations that were restricted to a toroidal subdivision (the "toroid") of the MGC. Similarly, neurons that responded selectively to antennal stimulation with (E,Z)-11,13-pentadecadienal, a more stable mimic of a second essential but chemically unstable pheromone component, (E,E,Z)-10,12,14-hexadecatrienal, had arborizations confined to a globular subdivision (the "cumulus") of the MGC situated more proximally to the antennal nerve input. One neuron that responded to both of these stimuli clearly had arborizations in both subdivisions of the MGC. These anatomically distinct subdivisions of the MGC thus appear also to be functionally separate regions of pheromone-processing neuropil.