Because of complexities in platelet serotonin uptake dynamics, we studied the influence of the time of day and year as well as the subject's age on uptake parameters. While the assay itself was quite reproducible, and the kinetic parameters of 5 HT uptake were stable over a few days, at a given time, within an individual, the variance was quite large when samples from different times of the day or year or from different individuals were compared. An inverse relationship between Vma x (moles/cell number/time) and platelet number was found in data from a group of individuals, suggesting regulation of Vm~ not at the level of uptake capacity per cell, but in a manner that somehow takes into consideration the number of platelets in the subject's plasma. Indeed, expressing Vm~ in a new way (called "'total V~."), not based on Vm~ , "per cell" or "per 10 7 cells" but for the total number of platelets in the volume of PRP used, greatly reduced the scatter in the between-individuals and across-time data. While Vm~ x (moles/cell number/time) exhibited only a trend toward reduction with age, for example, the decline in total Vmax with subject age was statistically significant. It is suggested that total V~,.~ (moles/time) may be a more physiologically relevant expression for an uptake function than Vm~ (moles/time/cell number).