Recent studies have suggested a role for P2 purinoceptors on vascular smooth muscle cells in the mechanism of renal autoregulation. Experiments were performed in anesthetized dogs (n = 9) to examine renal blood flow (RBF) autoregulatory efficiency before and after saturation of P2 purinoceptors with acute intra-arterial administration of ATP (1 mg/kg per min). Dogs were pretreated with the nitric oxide synthase inhibitor nitro-L-arginine (NLA) (50 μg/kg per min), to avoid endothelial P2 receptor-mediated effects on nitric oxide release caused by the intra-arterial ATP infusions. NLA treatment decreased RBF (5.3 ± 0.3 to 3.6 ± 0.2 ml/min per g) and sodium excretion (3.6 ± 0.4 to 0.9 ± 0.2 ml/min per g) without producing significant changes in GFR (0.92 ± 0.04 to 0.90 ± 0.06 ml/min per g) or RBF autoregulatory efficiency. ATP administration to NLA-treated dogs resulted in further decreases in RBF (2.8 ± 0.2 ml/min per g), GFR (0.58 ± 0.05 ml/min per g), and sodium excretion (0.6 ± 0.2 μmol/min per g). In addition, there was marked impairment of RBF autoregulatory efficiency during ATP infusion. The slopes of the arterial pressure-blood flow relationships at renal arterial pressures of >75 mmHg were significantly altered, from 0.003 ± 0.001 to 0.2 ± 0.002 ml/min per g per mmHg. Discontinuation of ATP infusion restored RBF autoregulatory efficiency. Norepinephrine (5 μg/kg per min) administration in these NLA-treated dogs decreased RBF (2.5 ± 0.3 ml/min per g; n = 4) to a similar extent, compared with ATP, but did not impair RBF autoregulation. These results support the hypothesis that P2 purinoceptors may be involved in mediating autoregulatory adjustments in renal vascular resistance.