Gastrointestinal blood loss has been studied following oral administration of the novel controlled-release acetylsalicylic acid tablet preparation Acetard and the instant-release acetylsalicylic acid tablet Magnecyl (Ph. Nord. 63). Acetard contains micro-encapsulated acetylsalicylic acid crystals having an in vitro release time of approximately 4 hours. The investigation was carried out as a two-part, randomized cross-over trial, and with a test dosage of either 1 g X 4 or 2 g X 2 per day, given to 10 and 14 male students, respectively, during two 5-day periods separated by a one week interval. The dosage of the plain formulation was maintained at 1 g X 4 daily in both parts of the investigation. Faeces were collected every 24 hours throughout the trial, covering a total of 4 weeks. Blood loss was measured using the 51Cr labelling technique. Acetard was found to cause statistically significantly less gastrointestinal blood loss as compared with the plain formulation, irrespective of whether Acetard was given twice or four times a day.