The effects of trandolapril on 24-h blood pressure in mild-to-moderate hypertensive patients of both sexes were investigated by conventional (clinic) and ambulatory recording in a double-blind study at two dosages. 1 mg (n = 14) and 2 mg (n = 13) once daily for 2 weeks. Both methods of measurement showed significant end-of-treatment decreases (p < 0.01 in all cases) in diastolic and systolic pressure in the 1− and 2-mg groups. Although intergroup differences were not significant, inspection of the mean changes from baseline in the eight 3-h periods constituting the 24-h profile showed that reductions were consistently greater in the 2-mg than in the 1-mg group, by 2 mm Hg diastolic blood pressure and 6 mm Hg systolic blood pressure. Values in the last segment of the placebo washout (46–48 h after the last active dose) showed that these reductions were well maintained, notably in the 2-mg group, with a minimal tendency to drift toward pretreatment levels. No effect was observed on the normal circadian blood pressure rhythm. Both doses were well tolerated. In conclusion, trandolapril is an effective. well-tolerated antihypertensive agent for once-daily dosing at either 1 or 2 mg.