The relationship between olfactory perception threshold and accompanying degrees of nasal obstruction, swelling, color and wetness was studied in eight subjects over a prolonged period. Olfactory acuity was impaired in two situations: a) in the presence of high degrees of nasal obstruction so that relatively little of the odorant-containing air could be brought into the nose, and b) in the absence of obstruction when the mucosa was relatively pale, dry and shrunken. During intermediate degrees of nasal obstruction when substantial amounts of air could be drawn into the nose relatively high degrees of swelling, redness and wetness favored olfactory acuity. Thus smelling was best performed in the presence of a red, swollen, wet mucosa as long as swelling and secretions were not of such a degree as to substantially block air passage. The evidence suggests that contact of odorant with nerve endings is facilitated by the greater warmth and humidifying effect of nasal engorgement during hyperfunction of the ‘air-conditioning’ apparatus of the nose. Submitted on March 14, 1960