Results of studies on the accuracy of the resistive index as a predictor of acute renal transplant rejection have varied widely. Clinical evaluations are limited by the inability to control the numerous coincidental factors that affect vascular resistance. We performed a controlled study in dogs to isolate the effects of acute tubular necrosis, cyclosporine toxicity, and acute rejection on the resistive index, and to compare them with a population of normal control subjects. By doing so, we hoped to identify the patterns of change in the resistive index over time and possibly explain the wide spectrum of resistive index data reported in the literature. Resistive index, a parameter calculated from relative systolic and diastolic velocity, indicates parenchymal resistance to perfusion. Since an increase in renal length also has been reported useful in predicting rejection, we studied changes in length in each of the isolated conditions. The normal control group (four dogs) had heterotopic autotransplantation with minimal cold ischemic time. The acute tubular necrosis group (six dogs) had heterotopic autotransplantation with 1 hr of warm ischemic time. The cyclosporine toxicity group (four dogs) was allowed approximately 3 months to heal from heterotopic autotransplantation. Very high (toxic) doses of cyclosporine were then administered. The acute rejection group (five dogs) had heterotopic allografting with minimal cold ischemic time. No medications were administered. In all groups, the abnormalities induced were confirmed by biopsy. Creatinine levels were also used to monitor cyclosporine toxicity. In the normal control and acute tubular necrosis groups, resistive index increased immediately after surgery, returning to baseline within 10 days. Renal length increased slightly in both groups, but the duration of increase was longer in the acute tubular necrosis group. No significant change in resistive index or renal length was seen in the cyclosporine toxicity group. In the acute rejection group, an initial decrease in resistive index during the mild to moderate phase was followed by a rapidly progressive increase with worsening rejection. Renal length increased progressively beginning immediately after surgery. Our study determined the patterns of change in resistance and renal length over time as caused by the isolated pathologic states. Our finding that vascular resistance decreased in mild to moderate acute rejection was unexpected, since almost all the literature reports resistive index elevation. This may explain some of the conflicting results obtained in Doppler investigations of rejection. Our results on renal length reinforce the positive clinical reports of its predictive value in rejection.