Renal blood flow (RBF) is markedly reduced in kidneys with unilateral ureteral obstruction (UUO), possibly due to vascular constriction. Whether obstructive nephropathy is associated with impaired RBF autoregulation is unknown. We therefore investigated RBF autoregulation in obstructed and contralateral unobstructed rat kidneys during and following release of 24 h and 6 days of UUO, using stepwise reduction of renal arterial pressure and electromagnetic recording of RBF. The lower pressure limit of autoregulation was increased and the maximal vasodilatory ability in response to infusion of acetylcholine into the renal artery was reduced only in the unobstructed kidney at 24 h of UUO. Thus, we conclude that the vasodilatory reactions to both these maneuvres, previously observed to be markedly reduced during acutely elevated ureteral pressure (Hope & Clausen 1982), were reestablished: In the obstructed kidneys in less than 24 h (RBF approximately 70% of control) and in the contralateral unobstructed kidneys in less than 6 days (RBF approximately 140% of control). Infusion of the thromboxane A2 (TXA) synthetase inhibitors imidazole and 3-ethyl pyridine in controls and at 24 h and 6 days of UUO did not produce renal vasodilation. These results do not support the suggestion that TXA contributes directly to the increase in renal vascular resistance observed during or following release of UUO in the rat.