To compare the long term effects of two low doses of aspirin taken daily, we performed a placebo-controlled cross-over study in 19 normal subjects. Aspirin 10 mg daily for 3 weeks caused a significant inhibition by 61 +/- 12 percent of platelet thromboxane B2 generation but had no effect on the Simplate bleeding time. Aspirin 30 mg daily for 3 weeks reduced thromboxane B2 production by 94 +/- 5 percent and caused a significant prolongation of the bleeding time, 1.6 times the control value. No cumulative inhibitory effects were observed after the first week of treatment. Both doses did not effect the urinary excretion of 6-keto-PGF1 alpha.