The pharmacokinetics and haemodynamic effects of infused urapidil and an infusion-capsule combination were followed to study the correlation between the serum urapidil level and the blood pressure. Prior to urapidil administration, basal blood pressure and heart rate were measured for 16 h in 12 male hypertensive patients. Six patients received infusions lasting for 4 h of urapidil 10, 2.5 and 5 mg/h. Six patients were infused with urapidil 10 mg/h for 4 h and 2 h after the end of the infusion each took a 60-mg capsule. After a 5 day washout period the procedures were crossed over. A maximum serum urapidil level of 625 +/- 232 ng/ml was achieved at the end of the 10 mg/h infusion, when the fall in blood pressure was 37/21 mmHg. During the 2.5 and 5 mg/h infusions the serum urapidil level was 330 and 420 ng/ml, respectively, and the corresponding decreases in blood pressure were 28/16 mmHg and 31/8 mmHg. Although the urapidil concentration 1 hour after beginning the infusion was only 184 +/- 89 ng/ml a near maximal blood pressure decrease had already occurred 33 +/- 9/20 +/- 8 mmHg, whereas, 1 h after the end of the infusion the reduction in blood pressure was only 10 +/- 12/3 +/- 8 mm, with a urapidil concentration of 358 +/- 120 ng/ml. During the plateau phases of both the infusion and infusion-capsule treatments the falls in blood pressure followed the serum urapidil levels. Only in the initial rising and final falling phases of the treatments were the pharmacodynamics and pharmacokinetics of urapidil not correlated.