Studies have shown that aspirin may decrease blood pressure when given at bedtime but not when administered on awakening. However, until now, a biologically plausible mechanism of this striking phenomenon was not revealed. We investigated the effect of 100 mg of aspirin administered at bedtime compared with administration on awakening on plasma renin activity and aldosterone levels over 24 hours and excretion of cortisol and catecholamines in 24-hour urine samples. A randomized, placebo-controlled, double-blind, crossover trial was performed in 16 grade 1 hypertensive subjects. During 2 periods of 2 weeks separated by a 4-week washout period, participants used aspirin both at morning and at night, which was blinded with placebo. After both periods, subjects were admitted for 24 hours to measure the aforementioned parameters. Aspirin intake at bedtime compared with on awakening reduced average (24-hour) plasma renin activity by 0.08 microg/L per hour (95% CI: 0.03 to 0.13 microg/L per hour; P=0.003) without affecting aldosterone levels (95% CI: -0.01 to 0.01 nmol/L; P=0.93). Cortisol excretion in 24-hour urine was 52 nmol/24 hours (95% CI: 5 to 99 nmol/24 hours; P=0.05) lower, and dopamine and norepinephrine excretions were 0.25 micromol/24 hours (95% CI: 0.01 to 0.48 micromol/24 hours; P=0.04) and 0.22 micromol/24 hours (95% CI: -0.03 to 0.46 micromol/24 hours; P=0.02) lower in patients treated with bedtime aspirin. In conclusion, aspirin taken at bedtime compared with on awakening significantly diminished 24-hour plasma renin activity and excretion of cortisol, dopamine, and norepinephrine in 24-hour urine. Decreased activity of these pressor systems forms a biologically plausible explanation for the finding that aspirin at night may reduce blood pressure, whereas aspirin at morning does not.