The effects of administered thromboxanes on the intact, normal rat kidney were studied. Euvolemic male rats received intraarterial infusions of thromboxane B2 (TXB2) and the stable thromboxane A2 analog (U46619) and the effects on renal function were investigated, using glomerular micropuncture and whole-kidney clearance techniques. Both TXB2 and U46619 were renal vasoconstrictors and lowered GFR by reducing renal plasma flow rate; U46619 was much more potent that TXB2. Neither agent caused any marked change in the glomerular capillary ultrafiltration coefficient (Kf). Thus in the rat, the thromboxanes reduce filtration rate by increasing renal vascular resistance and without exerting a marked influence on Kf.