In order to examine the intracellular thromboxane A<sub>2</sub> (TXA<sub>2</sub>) signal transduction system in platelets of patients with nephrotic syndrome, we measured the levels of TXA<sub>2</sub> metabolites in urine and blood and platelet calcium ion level as a result of STA2, an analog of STA<sub>2</sub> (9,11-dimethylmethano-11 12-methano-TXA<sub>2</sub>) stimulation, and obtained the following results: (1) In pediatric patients with nephrotic syndrome, urinary thromboxane B<sub>2</sub> (TXB<sub>2</sub>) and 11-dehydro-TXB<sub>2</sub> excretion were signficantly higher in the onset and relapse groups compared to the remission and control groups. (2) The blood 11-dehydro-TXB<sub>2</sub> level in the onset group was significantly higher than those in the remission and control groups. (3) Platelet calcium concentrations due to STA<sub>2</sub> stimulation were significantly increased in the onset, relapse and remission groups compared to the control group. These findings suggest activation of the TXA<sub>2</sub> signal transduction system in platelets of pediatric patients with nephrotic syndrome.