Abstract Objective—To investigate the effects of IM administration of acepromazine on indices of relative renal blood flow and glomerular filtration rate (GFR) by means of scintigraphy, as well as the effects on physiologic, hematologic, and serum biochemical variables in anesthetized dogs, compared with effects of administration of saline. Animals—6 healthy Beagles. Procedure—Acepromazine (0.1 mg/kg) or physiologic saline (0.9 NaCl) solution was administered IM 30 minutes prior to induction of anesthesia with thiopentone; anesthesia was maintained with inspired isoflurane for 2.25 hours. Blood gases and circulatory and ventilatory variables were monitored. Renal function was evaluated by scintigraphic measurements of GFR and relative renal blood flow and analyses of serum and urine. Statistical analyses used ANOVA or Friedman ANOVA. Results—Values of relative renal blood flow and GFR remained high despite low blood pressures. After administration of acepromazine, mean ± SD arterial blood pressure was 66 ± 8 mm Hg during anesthesia; this value was below the threshold (80 mm Hg) for renal autoregulation of GFR. In comparison, mean arterial blood pressure after administration of saline was significantly higher (87 ± 13 mm Hg). However, between treatments, there were no significant differences in GFR, relative renal blood flow, or other indices of renal function. Conclusions and Clinical Relevance—Measurements of renal function and blood flow in dogs during anesthesia with thiopentone and isoflurane did not differ significantly between treatments, which suggested that acepromazine protects renal function despite inducing reduction in blood pressure, compared with effects of administration of saline. ( Am J Vet Res 2003; 64:590–598)