Objective: to compare the effect of aspirin on blood pressure in two different diurnal times (morning and night) in women at preeclampsia risk. Method: 64 pregnant women randomly assigned to 4 groups. Testing group 1 (100 mg /day aspirin after awakening), testing group2 (100mg/day aspirin at bed time), control group1( placebo after awakening) and control group2 ( placebo at bed time). Inclusion criteria: women at higher risk for gestational hypertension and preeclampsia such as women with family history of gestational hypertension and preeclampsia, history of spontaneous abortion, obesity, adolescent or middle age nulliporous pregnancy (less than 18 or more than 35 years). Exclusion criteria: any needs to steroidal or non steroidal or hypertensive drugs, multiple pregnancy, any cardiovascular disease, chronic gastroenteritis and adrenal disease, asthma, hemorrhage disease, diabetes or any other endocrine disease such as hyperthyroidism, hematological disease and intolerance to devise (Ambulatory blood pressure monitoring), abortion, preeclampsia or eclampsia happening in pregnancy, forgetting > 6 tablet in month. The study was started at 12 to16 weeks of gestation and continued until 32 weeks of gestation. Blood pressure was automatically monitored for 24 consecutive hours every 4 weeks from the day of recruitment until delivery.