MANY sensory systems have evolved signal detection capabilities that a
re limited only by the physical attributes of the stimulus(1). For exa
mple, 'hair' cells of the inner ear can detect displacements of atomic
dimensions(2). Likewise, both in vertebrates and in invertebrates pho
toreceptors can detect a single photon(3,4). The olfactory stimulus al
so has a quantal unit, the single odorant molecule, Insects are report
edly able to detect a single pheromone molecule(5), whereas quantal re
sponses in vertebrate olfactory receptor cells have not been reported
yet. Psychophysical measurements indicate that a minimum of 50 odorant
molecules are necessary for human olfactory detection, suggesting tha
t an individual receptor may be activated ba a single odorant molecule
(6). We report here measurements of current fluctuations induced by od
orants that suggest a quantal event of about 03-1 pA, presumably trigg
ered by the binding of a single odorant molecule.