Action potential-generating properties of olfactory receptor neurons in the olfactory epithelium of the salamander, Ambystoma tigrinum, were studied in control animals, and 2 and 4 weeks after olfactory nerve transection. The threshold for impulse generation in response to injected current was extremely low (74 +/- 46 pA). In addition, the discharge frequencies of the receptor neurons were exquisitely sensitive to small increments of injected current. These high sensitivities may be characteristic of small neurons and stand in contrast to the much lower sensitivities reported for large neurons. The high sensitivity has important implications for the input-output functions of this cell. After nerve transection, both the threshold and the frequency sensitivity decreased. These changes appear to be associated with increased potassium conductance, suggested by prominent membrane rectification and reduced amplitudes of later membrane action potentials in the spike trains. The olfactory receptor neuron appears to be a favorable model for exploring these properties.