Abstract: Comparative study of enalapril and captopril was carried out on fifty patients with mild and moderate essential hypertension（EH）.Treatments were continued for 4 weeks in a single-blind,randomized,parallel design.Total effective rate in two groups were 60.0％ and 65.2％ respectively（P＞0.05）.Five patients in each group were studied by 24h-ABPM.In enalapril group the day-time BP was controlled satisfactorily（P＜0.05～0.01）but not in captopril group,in which the late night and early morning BP were close to the pretreatment level（P＞0.05）.We recommended that in patients with nocturnal hypertension,the long durable effective agent （such as enalapril 2／d） should be prefered.In patients without nocturnal hypertension the short halflife agent（captopril 2～3／d） seems to be the first choice.Because of a morning arouse of BP,it is recommened that captopril should be taken before getting up.