Ocimum basilicum L. (OBL), sweet basil, is a medicinal herb used in traditional Chinese medicine to treat cardiovascular diseases including hypertension. The objective of the study was to investigate the possible antihypertensive effects of OBL extract in renovascular hypertensive rats. The two-kidney one-clip (2K1C) Goldblatt model of renovascular hypertension was used in Wistar rats. Rats were randomized into sham, untreated 2K1C, captopril- (30 mg kg(-1) per day orally) and OBL- (100, 200, 400 mg kg(-1) per day orally) (low (L)-, medium (M)-, high (H)-OBL) treated 2K1C groups (n=10-12 per group), followed up for 4 weeks. Blood pressure, heart weight/body weight, plasma angiotensin-II and endothelin (ET)-1 were studied. OBL reduced systolic and diastolic blood pressure by about 20 and 15 mm Hg, respectively, compared with 35 and 22 mm Hg for captopril, from the lowest dose tested with no dose dependency. Cardiac hypertrophy was reduced from 3.6+/-0.7 mg g(-1) for untreated 2K1C to 3.0+/-0.6, 2.9+/-0.6 and 2.4+/-0.4 mg g(-1) for L-, M- and H-OBL, respectively, compared with 2.6+/-0.5 for sham and 3.1+/-0.4 mg g(-1) for captopril (P<0.05). Renal function was improved with captopril. Angiotensin was reduced to a lesser extent than with captopril. ET was reduced to lower concentrations (78+/-15, 80+/-22, 82+/-15 pg ml(-1) for L-, M-, H-OBL, respectively) than in sham (116+/-31 pg ml(-1)), untreated 2K1C (174+/-72 pg ml(-1)) or captopril (117+/-72 pg ml(-1)) groups. The effects of OBL on blood pressure, cardiac hypertrophy and ET, are consistent with an effect on ET-converting enzyme, and warrant further exploration.