We have developed a method for measurement of urinary 2,3-dinor-TXB2, which is a reflection of the actual rate of TXA2 synthesis by platelets in vivo. This determination was based on purification by high-pressure liquid chromatography and on measurement by RIA. After validation of the assay, we established the range of urinary Tx metabolites in healthy infants born at full-term during their first week of life. The 2,3-dinor-TXB2 levels were higher than in adults and declined gradually during the first seven days of life.