Eight patients with subarachnoid and/or intraventricular haemorrhage underwent continuous extraventricular drainage and cerebrospinal fluid production was estimated by modified open drainage. The patients were in Hunt & Hess grades 2-5 on admission. Drainage was instituted within 24 h after the last bleeding episode in seven patients and the duration of drainage was 3-37 days. The median amount of CSF that was drained in 24 h was 210 ml. CSF production rate was 0.10-0.55 ml/min (median 0.28 ml/min) and there was a great variation within as well as between patients. Thus there was a trend towards a reduction in CSF production compared to reported normal values for CSF production. Three of six surviving patients required a shunt. The possible role of reduction in CSF production rate in the modification of intracranial hypertension and hydrocephalus after subarachnoid haemorrhage is discussed.