1. Administration of the thromboxane A2 (TXA2) synthase inhibitor OKY 1581 to rats significantly increased the urine output elicited by the loop diuretic frusemide.

2. The administration of the TXA2 receptor antagonist SQ 29548 significantly increased the diuretic effect of frusemide.

3. Another TXA2 receptor antagonist L-640,035 increased significantly the diuretic effect of frusemide.

4. Both the sodium excretion rate and urine osmolar excretion rate were significantly increased in rats treated with the TXA2 synthase inhibitor OKY 1581 and frusemide.

5. The data suggest that TXA2 is released during frusemide-induced diuresis in rats, and the released TXA2 has an opposing antidiuretic effect.

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