The circadian rhythm of serotonin active transport in human platelets was investigated in ten healthy men, aged 27–35 years. Blood was collected at 08.00, 14.00, 20.00, 02.00 and 08.00 hours the next morning. Simultaneous evaluation of the mean platelet volume, platelet distribution width, platelet distribution skewness and platelet number in whole blood was performed. K _m and V _max for serotonin transport varied considerably among individuals over 24 h. However, the mean values and distribution of these kinetic parameters were reduced at 02.00 hours. All platelet size or number parameters were stable and normal over 24 h; therefore, the reduction in mean K _m and V _max values at 02.00 h is not related to morphological platelet differences but either to platelet intrinsic factors or plasmatic variables. Knowledge of the affinity and capacity of serotonin transport throughout the diurnal cycle is important for future comparisons with depressed patients as well as other hormonal rhythms in patients and healthy humans.