A unique probe--biotinylated adenosine-5′-monophosphate (5′AMP-biotin)-- was used in transmission electron microscopic (TEM) studies to localize 5′AMP odorant binding sites on the dendrites of olfactory receptor neurons in the aesthetasc sensilla of the spiny lobster, Panulirus argus. This probe is capable of both binding to and exciting 5′AMP- sensitive olfactory receptor neurons, as revealed through biochemical and electrophysiological assays. TEM studies showed that 5′AMP-biotin binding sites are distributed along the entire dendritic region that is exposed to odorants, including the transitional zone (between the inner and outer dendritic segments, including the ciliary segment) and all of the outer dendritic segment. The density of 5′AMP binding sites per micron2 of membrane is similar along the length of the olfactory dendrite. However, the relative number of 5′AMP-biotin binding sites per micron2 of sensillar area diminishes in the distal 30% of the aesthetasc due to a decrease in the amount of dendritic membrane in that region. The distribution of these 5′AMP binding sites is therefore much more extensive than that of enzymes that inactivate 5′AMP-- 5′ectonucleotidase/phosphatase--which are restricted to the transitional zone (Gleeson et al., 1991). Taken together, these results suggest that 5′AMP-biotin is labeling 5′AMP-specific olfactory receptor sites that are located along the entire outer dendritic segment and that can be coupled to olfactory transduction. This study represents the first in situ localization of specific olfactory receptor sites using a specific, functionally defined ligand.