A study was undertaken to investigate the interactions between aspirin and ascorbic acid in guinea pigs. Animals received by gastric intubation either a single dose of radiolabelled ascorbic acid alone or ascorbic acid with aspirin and the exhalation of CO2 was monitored for 400 min following administration. Animals receiving the vitamin only reached plasma peak levels within 90 min following administration while coadministration of the vitamin with aspirin, not only resulted in lower plasma peak levels, but also delayed their attainment until after 160 min. The bioavailability of ascorbic acid during the first 400 min was reduced by half following simultaneous administration of aspirin. The initial rate of exhalation of CO2 was decreased by 70% following coadministration of aspirin. These observations indicate that aspirin impairs the gastrointestinal absorption of ascorbic acid in guinea pigs, possibly by interfering with its active transport.