In order to investigate the blood pressure-heart rate interrelation and their circadian pattern in type I diabetes mellitus, we performed ambulatory blood pressure monitoring in 28 normotensive patients without clinical nephropathy divided in two groups. Group A consisted of 14 males with short-term DM (mean 2 years, mean age 28 +/- 6 years), group B consisted of 14 males with long-term DM (mean 12 years, mean age 31 +/- 7 years). Ambulatory blood pressure monitoring revealed significantly higher night heart rate in B group (74 +/- 13 l/min vs. 62 +/- 11 l/min in A, p < 0.01) and day diastolic blood pressure (83 +/- 9 mm Hg vs. 74 +/- 8 mm Hg in A, p < 0.01) and night diastolic blood pressure (73 +/- 10 mm Hg vs. 62 +/- 8 mm Hg in A, p < 0.01). The linear regression SBP/HR equation were significantly different (p < 0.02) (HR = 0.48 x SBP + 21, r = 0.40 in A vs. HR = 0.29 x SBP + 69, r = 0.28 in B). We concluded that type I diabetes duration has significant influence on diastolic blood pressure and heart rate even in patients without diabetic nephropathy and could be related to changed sensitivity of the baroreceptors.