Electro-olfactograms (EOGs) evoked by 8 odorants from frog olfactory epithelia during ciliary regrowth and during epithelial regeneration were analyzed. During ciliary regrowth following detergent-induced ciliary removal, EOG amplitudes initially increase proportionately with ciliary length. EOGs reach maximal amplitudes after 2 days of growth, when cilia are 40 micron long. Therefore olfactory transduction sites are located primarily on cilia rather than on the dendrite terminal and most of the receptor current enters through the proximal portion of the cilium. Zinc sulfate lavage of the nasal cavity causes selective necrosis of the receptor epithelium. During epithelial regeneration, EOGs increase linearly with time from 13 days after zinc lavage, the time of first cilium emergence, through 30 days. The rate of increase is different for different odorants. At 30 days and within a period of a few days, EOG amplitudes increase abruptly, then asymptote. Thus the development of receptors for different substances occurs at different rates and occurs in two steps. The transition between the two developmental states is coincident with arrival of receptor axon terminals at the central nervous system and with the immobilization of the ciliary contractile apparatus. Since there is continual generation of new receptor neurons throughout life, EOGs recorded in a normal nose reflect a complex combination of the differing receptor processes of cells of differing developmental stages.