The relationship of particle size of aspirin to gastrointestinal bleeding produced by this drug was examined in twelve healthy adults who received tablets containing the drug as fine particles, tablets containing relatively coarse particles of aspirin, and placebo tablets. Aspirin in the tablets containing the drug as fine particles dissolved and was absorbed from the gastrointestinal tract much more rapidly than aspirin in the tablets made with coarse particles. There was significantly more gastrointestinal bleeding during aspirin administration than during a control period in which no tablets were administered, but the difference in bleeding produced by the two types of aspirin tablets was not statistically significant. Placebo tablets did not cause gastrointestinal bleeding. Normal blood loss as determined du ring control periods and during placebo administration, respectively, showed a statistically significant correlation. However, subjects who bled rrwst in the control period did not necessarily bleed the most during aspirin administration.