Light microscopic immunohistochemistry coupled with freeze-substitution electron microscopic immunocytochemistry was used to localize alpha-subunits of G-proteins and type III adenylyl cyclase in developing rat olfactory epithelia. Some cilia immunoreacted with antibodies to GS alpha and type III adenylyl cyclase as early as prenatal day 15 (E15; E1 = sperm-positive), but immunolabelling with antibodies to Golf alpha was not observed until E16. From then on numbers of receptor cells with immunolabelled cilia increased for all three probes. Immunoreactivity for antibodies to the olfactory signal-transduction proteins tended to parallel cilium development, though Golf alpha lags somewhat behind. Newly formed cilia labelled along their lengths, whereas mature cilia labelled predominantly along their long distal parts. Dendritic knobs and ciliary necklaces showed little or no labelling. While at E22 most multiciliate cells immunolabelled with antibodies to Gs alpha, Golf alpha, and type III adenylyl cyclase, not all of these cells labelled with antibodies to olfactory marker protein. Olfactory axons immunoreacted more intensely than epithelial surface structures with antibodies to Gs alpha at E15; the reverse occurred by about E18. Immunoreactivity with antibodies to alpha-subunits of the G-proteins Go, Gq/G11, and Gi was also found as early as E15. Antibodies to Go alpha labelled receptor cell dendritic knobs and cilia during development only. Antibodies to Gi alpha labelled Bowman's glands, whereas those to Gq alpha/G11 alpha bound to receptor cell cilia and axons (primarily vomeronasal), and supporting cell microvilli. We propose that Gs is the predominant G protein in cilia of immature olfactory receptor cells, while Golf is predominant in cilia of mature cells. Axonal immunoreactivity for some G-protein antibodies suggests G-protein participation in processing of olfactory axon and/or axon terminal-bound signals.