Gastrointestinal side effects caused by naproxen and oxindanac (a developmental non-steroidal anti-inflammatory drug) were compared by combined endoscopy and determination of faecal blood loss in 16 healthy male volunteers in a randomized, double-blind, crossover study. Individual daily faecal blood loss was determined by means of 51Cr-labelled erythrocytes. Gastroduodenoscopy was performed before and after administration of naproxen, 750 mg/day, and oxindanac, 600 mg/day, for 1 week each. A washout period of at least 3 weeks was inserted between drug periods. Visual analogue scales (VAS) were used for endoscopic assessment of lesions and subjective complaints. Mean faecal blood loss increased from a base line 0.48 ml/24 h to 1.59 ml/24 h with naproxen (p less than 0.01) and from 0.56 ml/24 h to 1.31 ml/24 h with oxindanac (p less than 0.01). VAS scores for gastroduodenal lesions increased significantly with both drugs. Naproxen caused a significantly greater increase than oxindanac (p less than 0.05). There was no correlation between gastrointestinal blood loss and endoscopic findings. Subjective symptoms were correlated to faecal blood loss with naproxen, but not to endoscopic findings. No such correlations were observed for oxindanac. Naproxen caused a significant prolongation of bleeding time (p less than 0.01), whereas the increase caused by oxindanac was not significant (p = 0.09).