Unitary recordings of near threshold responses of receptor cells in the olfactory mucosa of the frog.

1. Receptor cell activity in the frog's eminentia olfactoria was recorded using metal-filled micro-electrodes. 2. Several units discharged spontaneously with a mean frequency lower than 0.2 spikes per sec, or were silent in periods of up to 5 min. The other units displayed spontaneous activities between 0.2 and 1.05 spikes per sec; their activity could be modelled with a Poisson process. 3. Near-threshold responses to odour stimulation were investigated, considering several stimulations within a small concentration range. Low concentration stimulations were sometimes followed by a response, sometimes not. The concept of response probability is introduced to describe this incertitude. 4. The distribution of the number of spikes in several odour trials at low concentrations showed a reasonable agreement with two types of Poisson distribution. 5. The findings are discussed in connexion with receptor cell sensitivity and the excitation of second order neurones in the bulb.