The effect of single dose (50 mg) Captopril (C) used either alone or associated to diuretics (50 mg hydrochlorothiazide -HCTI) in the treatment of mild-moderate essential arterial hypertension was studied in a multicentric study. Eighty eight patients were chosen. After a minimum of 4 month follow-up period 53.4% responded (BDP less than 95 mm Hg) to single dose C (group 1:47 patients), 89.77% to 50 mg C in single dose together with 50 mg HTIT (group 2: 32 patients), 95.45% of two 50 mg doses of C and 50 mg HCIT (group 3: 5 patients), and 97.72% responded to 3 doses of C and 50 mg of HCTI (group 4: 2 patients). The decrease in blood pressure values was statistically significant (p, 000, Wilcoxon test) in groups 1 and 2, having a mean decrease in blood pressure (BP) of 14%. In group 1 (n = 42) the SBP which initially was 165.72 +/- 11.32, decreased to 148.28 +/- 11.5 and the DBP decreased from 101.55 +/- 5.68 to 87.28 +/- 6.59. In group 2 (n = 32) the SBP decreased from 173.50 +/- 14.08 to 152.44 +/- 20.8 and the DBP from 103.34 +/- 5.29 to 87.47 +/- 6.39. The response to monotherapy could not be statistically correlated either to early essential hypertension or to the patients age. Treatment was discontinued in three cases due to the secondary effects, cough, ageusia and nervousness, showing the remaining patients a good tolerance. No changes were observed in the analytical parameters. This study shows the usefulness and tolerance of single dose C as the initial treatment of mild to moderate essential hypertension.