A controlled study of the acute effects on the gastric mucosa of acetylsalicylic acid in an alkaline and an acid environment was performed in healthy students. Gastric blood loss was determined by the Hemastix method and direct visual observations of the gastric mucosa were made with a gastrocamera. This study has shown that hemorrhagic lesions of the gastric mucosa are common in university students when aspirin tablets are ingested in the presence of a low intragastric pH. There were no reactions when the intragastric pH was alkaline. Gastrocamera photographs are presented to demonstrate the types of reactions which occurred. The findings suggest that it is the unionized acetylsalicylic acid rather than its sodium salt which is injurious to the gastric mucosa.