In this experiment the authors examined the nasal secretum of 36 normal subjects. Samples were taken at 10 a.m., 1 p.m., 4 p.m., 7 p.m., 10 p.m., 1 a.m., 4 a.m., for 3 days and 2 weeks later. SIgA, albumin and urea were measured in the secretum. The values obtained showed an increase in the three substances, when correlated to a different transudation during the 24 h, but the greater increase in SIgA must be attributed to an accentuated local production. Undoubtedly there exists a circadian modification in the production of SIgA, with higher values during the night than during the day.