In an experiment carried out on 6 [human] volunteers, measurement of total serum salicylate concentration was used to describe aspirin kinetics after a single 1500 mg oral dose. The availability of the drug was significantly better when it was administered at 600 h than at 1800 h or 2200 h. Also a significant time-dependence was found between the morning and evening values of both the serum half-life time of salicylate and ke1. The possible relation between these results and the appearance of undesirable side effects in patients should be examined.