Focal gastric mucosal blood flow was studied during aspirin injury by hydrogen gas clearance in a chambered segment model of canine gastric corpus. Measurements were made simultaneously every 15 minutes at ulcerated and nonulcerated areas 1.5 hours before, during (20 mM of aspirin in 150 mM of HCl for 1 hour), and 2 hours after exposure of the mucosa to topical aspirin. There was a highly significant decrease (p less than 0.001) in flow at the ulcerated areas 30 minutes after exposure to aspirin, coinciding with the appearance of focal mucosal pallor followed by subsequent hemorrhagic foci and ulceration. This was not followed by recovery to basal flow values. Blood flow to the non-ulcerated areas was significantly but less severely reduced than in the ulcerated areas (p less than 0.05) 90 minutes after exposure to aspirin. This was followed by recovery to basal levels. It is proposed that aspirin induces reduction of focal mucosal blood flow of varying degrees and that mucosal areas with flow reduced to below a "critical value" develop gross damage.