Tryptophan, 5-hydroxyindoleacetic acid and indoleacetic acid were measured in cerebrospinal fluid, taken during pneumoencephalography, from a large series of patients, the majority of whom were epileptics, most of them receiving anticonvulsants. CSF indoleacetic acid reflects CNS tyrptamine metabolism in the same way that CSF 5-hydroxyindoleacetic acid reflects CNS 5-hydroxytryptamine metabolism. Our data suggest that (i) the brain tryptophan content is an important factor in the control of both 5-hydroxytryptamine and tryptamine synthesis (ii) brain 5-hydroxytryptamine metabolism exhibits a U-shaped relationship with age (iii) the mean brain tryptophan content and rate of 5-hydroxytryptamine metabolism are greater for women than men (iv) indoleamine metabolism is unaffected in untreated epileptics compared with non-epileptics, but anticonvulsant drugs decrease the rate of 5-hydrosytryptamine metabolism.