As human blood platelets may be regarded as a reliable model for serotonergic synaptosome because of theirspecific biochemical mechanisms for uptake, storage, metabolism and release of amine, the whole blood measurement of serotonin (thatis, mainly platelet serotonin, since very little is detectable out-side the platelets) provides a peripheral model for the study of processes in the brain. 6" However, it is still an open question as to whether bloodplatelet parameters reflect those in the brain. The daily fluctuations ofbehaviour in affective disorders have stimulated us to search forcircadianism of serotonin.

We examined at five time points whole blood 5HT levels in psychiatric patients in hospital and control volunteers. The group of patients consisted of18 subjects (12 with depression and six with neurosis). They were diagnosed and treated in the Department of Psychiatry, Hamamatsu University School of Medicine. At the time the blood samples were taken, patients were in a depressive state, confirmed by a mean score of 18-7 (range 12-24) on the 24-item scale of Hamil-ton Depression Rating Scale. 8 None of them were receiving any drug, except for small does of bezodiazepines, for at least 10 days before blood was taken. Before entering the trial, the patients were in good physical health and gave voluntary consent to partici-pate. Ethical committee approval for this study was obtained from the Hamamatsu University School of Medicine. The control group matched for age and sex with the patients was made up of 30 volunteers, who showed no clinical signs of depression or organic disease, and were drug-free. Quantitative analysis of 5HT in the blood was performed by HPLC as described by Anderson et al. 9 Analytical recoveries were 85%(SD 4-5%, CV 5-6%). Amount and response for standards were linearly related. As shown in the figure, whole blood serotonin level was significantly reduced in