The effect of 0.05 and 0.03 mg/kg of intravenously administered lorazepam on the ventricular fluid pressure (VFP) was recorded continuously for 45–90 minutes in 13 wakeful spontaneously breathing unanaesthetized patients with hydrocephalus. The initial VFP was low in 11 patients with low-pressure hydrocephalus, and at the upper level of normal in 2 who had stenosis of the aqueduct. Lorazepam caused minute changes in VFP. The largest transient increases (7 and 16 torr) occurred in the two patients with the highest initial VFP. Blood acid-base balance, blood pressure, and heart rate remained unaltered. However, lorazepam caused such drowsiness that it was difficult to check the patients' level of consciousness. For this reason, intravenously administered lorazepam in a dosage of 0.03 mg/kg or more seems unsuitable for premedication in neurosurgical patients with brain disease.