Ketanserin is a new potent antiserotonergic drug which, unlike previous ones, is selective for S2-serotoninergic receptors and does not have an agonist action. A trial was carried out on medium-term treatment with ketanserin or propranolol in subjects suffering from mild to moderate hypertension. The trial was designed as a double-blind crossover randomized study comparing either ketanserin or propranolol with placebo. Thirteen patients completed the study, which was divided into two groups (A and B). Systolic (SBP), diastolic (DBP) and mean (MBP) blood pressures were measured by non-invasive, intermittent ambulatory monitoring performed using a Pressurometer II, from Del Mar Avionics. Heart rate was measured using a continuous electrocardiogram monitoring. Systolic blood pressure was significantly reduced both after ketanserin (A:11.1%; B:10.8%) and propranolol (A:11.7%; B:11.8%) but in group A its decrease was more pronounced after propranolol (P less than 0.01). Diastolic blood pressure was significantly reduced both after ketanserin (A:11.5%; B:11.1%) and propranolol (A:11.4%; B:11.9%), as was MBP (A:11.9%; B:11.8% for ketanserin and A:11.9%; B:11.9% for propranolol). The heart rate diminished significantly only after propranolol administration (P less than 0.01). Ambulatory monitoring showed a significant 24-h reduction of SBP after administration of propranolol (P less than 0.0025) and ketanserin (A:P less than 0.0025, B: P less than 0.005). Diastolic blood pressure was also significantly reduced after ketanserin (P less than 0.0005) and propranolol (A: P less than 0.0025, B: P less than 0.0005). The heart rate obtained by continuous electrocardiogram monitoring diminished significantly only after propranolol administration (P less than 0.0005). No significant changes of circadian behaviour of blood pressure were observed.