1. We investigated the effect of the haemostatic agent ethamsylate on aspirin-induced gastric mucosal bleeding. 2. Eighteen healthy subjects were studied three times: at the end of 48 h periods of treatment with (a) placebo, (b) aspirin 600 mg four times daily, (9 doses) and (c) aspirin 600 mg four times daily with each dose preceded by ethamsylate 500 mg. 3. At the end of each treatment period gastric mucosal bleeding into timed gastric washings was quantified using the orthotolidine reaction. 4. Aspirin increased bleeding from a rate on placebo of 1.2 microliters 10 min-1 geometric mean (95% confidence limits) (0.7-1.8) microliters 10 min-1 to 20.0 (11.6-34.2) microliters 10 min-1, (P less than 0.01). The rate of bleeding after aspirin preceded by ethamsylate [14.1 (8.5-23.4) microliters 10 min-1] was not significantly different from that after aspirin alone. 5. We conclude that ethamsylate does not reduce acute aspirin-induced gastric mucosal bleeding in healthy humans.