Urea, creatinine (24 cases) and albumin (22 cases) were determined in nasal secretum and in serum in 50 subjects. The correlation between the values in the secretum and in the serum significative only for urea and creatinine. The regression coefficient of urea was particularly high and we suggest that the values of urea in nasal secretum may be calculated as 1, 3 times its serum levels. This relationship may be useful to calculate the dilution of the samples of respiratory secretum collected by lavage, utilizing the ratio between the actual urea in the sample and its theorical value.