The study was designed to evaluate the urinary excretion of C-peptide and albumin, and urinary N-acetyl-beta-D-glucosaminidase (NAG) activity in juvenile borderline hypertensives. The second aim was to examine the relationship between these variables and ambulatory blood pressure level and variability. The study group consisted of 21 non-obese males consecutively chosen from patients with borderline hypertension, defined by sphygmanometer readings, examined in our outpatient clinic. All subjects collected separately their day-time and night-time urines during the period of ambulatory blood pressure monitoring. In 16 patients, who were considered to have "sustained" borderline hypertension, both 24-h urinary C-peptide excretion and 24-h UAE were significantly increased in comparison to those of the controls, while NAG activity did not differ significantly between the two groups. UAE was significantly lower at night than during the day in both borderline hypertensives and controls. Twenty-four-hour UAE in borderline hypertensives correlated significantly with the ambulatory blood pressure variability, but not with the average blood pressure level. These results suggest that the 24-h insulin secretion rate estimated by means of urinary C-peptide excretion is significantly increased in "sustained" borderline hypertensives. Elevated UAE in juvenile borderline hypertensives can be explained by a possible direct effect of systemic blood pressure variability on albuminuria.