Few reports, especially experimental ones indicate cause-effect connection between melatonin (Mel) and blood pressure. The aim of the study was to assess connection between a daily profile of Mel concentration in saliva and 24-hour cycle of blood pressure in 22 patients with moderate hypertension (12 men, 10 women), 20 patients with severe hypertension (9 men, 11 women) and 25 healthy subjects with normal blood pressure (13 men, 12 women). The age of studied subjects oscillated between 37 and 46 years. Concentrations of Mel were assessed in saliva collected during the day with intervals of 3 hours with the use of RIA method. Blood pressure assessment was done at the same time. A significant decrease of mesor's value and amplitude of Mel rhythm without essential changes in acrophase's value were observed in men and women with severe hypertension. Mean concentrations of Mel in saliva were significantly decreased from 8 pm to 5 am. Significant, negative correlation between nighttime Mel concentrations in saliva and both systolic and diastolic blood pressure was shown in patients with moderate and severe hypertension. Correlation coefficients values were in general higher in patients with severe hypertension. Results of our studies seem to confirm the conception that decreased Mel secretion can be one of the causes of hypertension.