To shed light on the role of thromboxane A2 (TXA2) in renal injury, we evaluated the effects of S-1452, a TXA2 receptor antagonist, on rats with streptozotocin (STZ)-induced diabetic nephropathy and murine lupus nephritis. In STZ diabetes rats (n = 6), urinary protein excretion significantly increased from 8 weeks and was about 5 times as much as that in normal rats at 10 weeks after induction of diabetes. In S-1452-treated rats (n = 6), increase in urinary protein was rarely observed and was significantly inhibited at 8 and 10 weeks after induction of diabetes. In (NZB x NZW)F1 mice, no proteinuria was detected in vehicle controls (n = 20) and S-1452-treated mice (n = 20) from 0 to 8 weeks after initiation of S-1452 treatment. Proteinuria was observed in 3, 7 and 8 mice in the control group, and 0, 2 and 5 mice in the S-1452 group at 12, 16 and 20 weeks after initiation of S-1452 treatment, respectively. Proteinuria developed more slowly in S-1452-treated mice than in vehicle controls. In conclusion, TXA2 receptor antagonist, S-1452, suppresses the progression of renal injury.