Twenty-four adult volunteers were inoculated with nasal drops containing a coronavirus of 229E serotype to determine the differences in the clinical and physiological reactions which occur between clinically infected, sub-clinically infected and non-infected individuals. Thirteen volunteers were clinically infected, 8 had sub-clinical infections and 3 were uninfected. Nasal airway resistance and the temperature of the nasal mucosa increased in all infected subjects both with and without symptoms: the core temperature increased also but to a lesser extent. Mucosal blood flow and nasal secretion increased only in those with symptoms. The albumin content of the nasal secretion increased in the clinically infected, suggesting that it was derived, partially at least, from the circulation. The nasal cycle of variation in airway resistance between the two sides of the nose was observed in all three groups but increased only in those clinically infected.