Absence of the normal 10% reduction of ambulatory blood pressure (ABP) from day to night is associated with morbidity and mortality. The cause of this reduction (or its absence) is not completely understood. We studied excretion of norepinephrine (NE) and epinephrine (EPI) on the same day ABP monitoring was performed in 37 healthy volunteers aged 52±7years, with body mass index (BMI) 25±4 Kg/m2, and 24-hour ABP 130±11/82±9 mmHg. Overall SBP decreased from day to night 134±13 to 117±10 mmHg, diastolic blood pressure (DBP) from 86±9 to 72±8 mmHg, and heart rate (HR) from 80±10 to 70±9 beats per minute (bpm). Urinary NE and EPI excretion decreased (p<0.005) from 19±10 to 10±6 μmol/mol creatinine, and from 4±2 to 0.8±0.4 μmol/mol creatinine, respectively. Awake or asleep SBP was not correlated to the corresponding NE excretion (r=-0.2, and 0.19). We defined SBP dippers (n=22) as those with a night/day ratio (NDR) of <0.9. There were no differences in age, gender, BMI, 24-hour ABP, daytime DBP, sleep ABP, daytime or sleep HR, or daytime or overnight NE or EPI excretion. On the other hand awake SBP 129±11 mmHg was lower, p<0.05, and NDR DBP 0.89±0.05, NDR HR 0.90±0.05, and NDR NE 0.66±2.7, were significantly higher p<0.05, in the non-dippers (n=15) , than in dippers 138±17mmHg, 0.81±0.05, 0.86±0.8, 0.49±0.18, respectively. In a stepwise multiple regression analysis model with SBP NDR as the dependent variable, awake SBP, p=0.0008, and NDR NE p=0.03, were significant predictors, whereas age and BMI were excluded from the model. In conclusion although day and night blood pressure and urinary NE are not related, the nocturnal decrease of SBP is closely related to the degree of reduction in sympathetic activity as reflected by urinary NE excretion on the same day.