Mean adrenaline concentration in cerebrospinal fluid measured by a sensitive and specific isotope-derivative assay was significantly lower in 15 depressed patients during illness compared with 18 control subjects. At the time of recovery cerebrospinal adrenaline levels had increased markedly to normal levels. Cerebrospinal fluid noradrenaline did not differ in patients compared with controls. The present findings suggest that adrenaline as a neurotransmitter may be involved in affective disorders.