One and five-tenths milligrams of [3H8]thromboxane B2 (12.2 Ci/mol) was infused at a maximum rate of 6.4 microgram/min into a healthy male volunteer. Seventy-four per cent of the infused radioactivity was recovered in the urine within 13 h. Urinary metabolites of thromboxane B2 were isolated by reversed phase partition chromatography and high performance liquid chromatography. The major urinary metabolite was found to represent approximately 16.8% of the total radioactivity in the urine. The structure of this metabolite was shown by gas chromatography-mass spectrometry to be 2,3-dinor-thromboxane B2, the product of a single step of beta-oxidation. The rate of excretion of infused radioactivity and the relative percentage represented by 2,3-dinor-thromboxane B2 in the urine of man are very similar and the major urinary metabolite identical to that previously found by us in the non-human primate.