In order to study the nasal response to serotonin, 14 normal persons, in a double-blind study, were provoked in the nose with serotonin and histamine. Itching and the number of sneezes were noted, the amount of secretion measured, and nasal airway resistance recorded by active posterior rhinomanometry. Serotonin induced significant nasal itching, sneezing and hypersecretion, similar to the effects of histamine. The effect of serotonin on nasal airway resistance, on the other hand, was slight (+ 10%) and insignificant in contrast to that of histamine in equipotent doses (+ 48%) (P less than 0.001). In conclusion, we have shown that serotonin provocation can induce a rhinitis response in the human nose. The nasal symptoms suggest an effect on sensory nerves with reflex-induced sneezing and hypersecretion, while there appears to be little direct effect on capacitance vessels. The possible role of serotonin as a mediator of rhinitis remains speculative.