The antihypertensive agent dilevalol, a stereoisomer of labetalol, causes vasodilatation by combined beta-adrenergic blockade and direct beta 2-receptor agonism. The acute effect of a single oral dose of 400 mg dilevalol on cerebral blood flow (CBF), and its response to induced changes in arterial carbon dioxide tension (PaCO2), and the cerebral metabolic rate of oxygen (CMRO2) were studied after 1, 2, and 4 h in eight normotensive volunteers. Serum dilevalol 2 h after oral intake ranged between 22 and 49 ng/ml. Mean arterial blood pressure was reduced from 93 +/- 8 to 86 +/- 8 mm Hg (p less than 0.01). An analysis of variance of the successive differences of CBF during the study showed that dilevalol altered CBF significantly. After 1 h CBF had decreased by 4.4% (p less than 0.05), whereas the 2 and 4 h values were similar to the pretreatment level. CMRO2 showed a tendency for reduction (p = 0.086). The CBF response to changes in PaCO2 remained unchanged. It is concluded that a single dose of dilevalol induces a slight and short-lasting decrease in CBF, probably due to a direct effect on brain metabolism.