The present study was undertaken in 8 healthy volunteers to examine the effect of a clinically relevant dose of nimodipine (NIM) (15 and 30 microgram/kg/h) on CBF, its CO2 reactivity, and CMRO2. Mean arterial blood pressure (MABP) was measured intra-arterially. Regional CBF was measured by SPECT of inhaled Xenon-133. During the CO2 reactivity tests changes in CBF were estimated by the arterio-venous-oxygen-difference method. Median CBF was 52 ml/100 g/min (48-53) with a normal regional distribution, and median baseline MABP was 96 mmHg (92-99). MABP was slightly reduced, by 8 mmHg (7-9), and 9 mmHg (4-11) after infusion of NIM for 2 and 4 hours, respectively. CBF, however, remained constant, although correction for changes in PaCO2, revealed a slight increase after 4 hours (p = 0.08). CMRO2 was 3.5 ml/100g/min (3.2-3.5) and was not changed by the infusion of NIM. At arterial CO2 tensions ranging from 4.0 to 6.5 Kpa the CO2 reactivity was 3.0% CBF/0.1 kPa (2.6-3.7) and decreased significantly to 2.6% CBF/0.1 kPa (1.8-3.2) after the infusion of NIM for 3 hours (p = 0.02). The median slope of the LnCBFsat/PaCO2 relationship was 1.5 at baseline compared to 1.3 after NIM (p less than 0.01). No side effects were observed. The present study shows a decreased CO2 of the cerebral vessels and a maintained coupling of CBF and CMRO2 during the infusion of nimodipine.