MANY sensory systems have evolved signal detection capabilities that are limited only by the physical attributes of the stimulus(1). For example, ’hair’ cells of the inner ear can detect displacements of atomic dimensions(2). Likewise, both in vertebrates and in invertebrates photoreceptors can detect a single photon(3,4). The olfactory stimulus also has a quantal unit, the single odorant molecule, Insects are reportedly able to detect a single pheromone molecule(5), whereas quantal responses 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 that an individual receptor may be activated ba a single odorant molecule(6). We report here measurements of current fluctuations induced by odorants that suggest a quantal event of about 03-1 pA, presumably triggered by the binding of a single odorant molecule.