To explore the regularity of nyctohemeral rhythm of blood pressure, the ambulatory blood pressure was monitored in 25 patients of mild and moderate primary hypertension and 25 patients of renal hypertension. The results showed that the systolic and diastolic blood pressure was decreased by 11.4±7.6%(2.18±1.39 kPa) and 13.5±9.1%(1.67±1.11 kPa) respectively at night in patients with mild and moderate primary hypertension. For patients of renal hypertension, blood pressure was decreased by 2.6±7.0%(0.58±1.45kPa) and 3.7±8.2%(0.46±1.06 kPa) respectively at night. The differences between two types of hypertension were statistically significant. It can be concluded that there is a nyctohemeral rhythm of blood pressure in mild and moderate primary hypertension, but the rhythm of blood pressure is less prominent in renal hypertension. This characteristic may help to distinguish mild and moderate primary hypertension from renal hypertension.