We examined the haemorheological determinants in a group of 52 hypertensives (diastolic basal pressure greater than 100 mmHg) subdivided in relation to the clinical stage (I and II or III according to the WHO classification) and in a group of 50 normal controls, matched for age and sex. From this study, it is evident that only plasma viscosity, fibrinogen and the fibrinogen/albumin ratio are able to distinguish normals from hypertensives. It is also clear that the clinical stage does not influence the haemorheological findings. These haemorheological factors were also measured in 3 groups of hypertensives treated with either timolol (20 mg/day), metoprolol (200 mg/day) or oxprenolol (160 mg/day). After a basal evaluation, measurements were performed, at regular intervals of 10 days, for a period of 30 days. The only significant changes were obtained using timolol; metoprolol and oxprenolol, did not affect these haemorheological parameters.