The aim of our work was to develop an assay for the determination of proguanylin in human blood, and investigate its levels in healthy volunteers and donors suffer from hypertension often accompanied by body sodium accumulation and plasma volume expansion. We developed and evaluated the sandwich ELISA method for the quantitative determination of human proguanylin in serum samples. We conducted also the pilot study on individuals with hypertension and oh healthy probands and measured proguanylin serum levels, serum and urine sodium and creatinine levels. In the study on 256 healthy volunteers we demonstrated that women have significantly higher values of proguanylin than men (medians 12.7 vs. 9.6 ng/ml, p < 0.01) and proguanylin values increased with age of individuals (p < 0.01). Futhermore, we tested 17 individuals with hypertension and found that probands with anamnesi of hypertension had higher proguanylin values than healthy individuals from the first study (medians 16.2 vs. 11.3 ng/ml, p < 0.01). Both of groups did not differ in sex or age. Proguanylin values correlated with the systolic blood pressure (r = 0.41, p < 0.01), sodium fraction excretion (r = 0.72, p < 0.01) and serum sodium (r = -0.39, p < 0.01). No significant correlation we found with serum proguanylin and creatinine. In the group of 9 healthy probands we demonstrated the existence of a diurnal rhythm of proguanylin with its maximum in the evening hours (between 6-10 p.m.). The pilot study supports the hypothesis about the role of proguanylin in sodium metabolism and its possible importance for hypertension disorder. Further research is necessary to confirm our findings in individuals with hypertension with different medication in order to assess proguanylin value as a risk predictor of accelerated hypertension, and to classify individuals with hypertension for variuos types of diuretic therapy.