The effects of hypocapnic alkalosis on the vasodilating action of nicardipine were studied in 6 patients after cerebral arterial aneurysm surgery. Each patient served as his/her own control during the 6 steps of the study. T0: baseline; T1: hypocapnic alkalosis alone (PaCO2: 3.5 kPa); T2: hypocapnic alkalosis and bolus injection of nicardipine (30 micrograms.kg-1 i.v.); T3: hypocapnic alkalosis and continuous 60 min infusion of nicardipine (0.5 microgram.kg-1.min-1), T4: determination of the infusion rate required to neutralize the effect of hypocapnic alkalosis; T5: same continuous dose of nicardipine as in T4 but reversal of hypocapnic alkalosis. Hypocapnic alkalosis alone caused a significant increase in the systemic vascular resistance index by 20% (T1). The bolus injection of nicardipine reversed this first effect (T2). The continuous infusion of nicardipine in T3 was insufficient to cancel the haemodynamic effect of hypocapnic alkalosis. During T4 the plasma levels required to neutralize completely the effect of hypocapnic alkalosis were twice those at T3. Normalization of the PaCO2 in step T5 induced a significant fall in the systemic vascular resistance index by 27.5% as compared with T0. In this study hypocapnic alkalosis modified the relationship between plasma levels of nicardipine and its expected vasoactive effects. This interaction was reversible.