Homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA) and tryptophan (TRP) were measured in the CSF obtained from the basal cistern of 20 patients, who had undergone surgical obliteration of bleeding aneurysms within 3 days after subarachnoid haemorrhage (SAH). The concentrations of these substances, which were assayed by high performance liquid chromatography (HPLC), were the highest on days 3 or 4, and thereafter showed a gradual decrease with an increase in time. The cisternal CSF in patients who had severe preoperative clinical grades of SAH or vasospasm contained relatively higher concentrations of HVA, 5-HIAA and TRP than those contained in patients who did not. It is proposed that SAH stimulates the release of monoamines from the brain with the resultant extracellular accumulation of their metabolites and their diffusion into CSF during the acute stage. This stimulatory effect of SAH on the brain monoamine system may be consistent with those previously reported in cases of cerebral haemorrhage or infarction.