The diurnal rhythm but not absolute values of platelet serotonin (5-HT) was modified in untreated unipolar depressives. When allowance was made for the pronounced seasonal fluctuations in platelet 5-HT the difference was even more significant: in the first half of the year 5-HT was higher at 8 a.m. than at 4 p.m. in controls but the reverse was found in unipolar depressives. Parallel measurements of platelet 5-HT uptake and monoamineoxidase activity in the same patients showed no differences from controls, indicating that these regulatory factors did not determine the altered rhythmicity of platelet 5-HT. However plasma free tryptophan rhythmicity was also altered. These observations of altered rhythms but not concentrations of two peripheral serotoninergic parameters are of interest in the light of the concept of a desynchronization phenomenon underlying depressive states.