The effect of a 4.5 mL/h Ringer infusion on the recovery from a unilateral 40-min renal artery occlusion was investigated in Sprague-Dawley rats. The inulin clearance measured in the experimental kidney 24 and 48 h after the insult in control animals that did not receive the Ringer infusion was 0.14 +/- 0.10 (mean +/- SE) and 0.11 +/- 0.05 mL/min, respectively. In animals that received 24 h of Ringer infusion begun at the time of the renal artery occlusion the inulin clearance was 0.81 +/- 0.07 mL/min, a value significantly higher than either of the control groups (p less than .05). If, however, the Ringer infusion was stopped at 24 h and the inulin clearance measured at 48 h, it had decreased significantly (0.27 +/- 0.09 mL/min) and was no longer greater than the control groups. Similarly, if the infusion was continued for 48 h there was no longer a significant difference between the inulin clearance (0.37 +/- 0.11), when compared with 48 h of no infusion (0.11 +/- 0.05). The histology of the different groups corresponded with the functional data. We conclude that 24 h of Ringer infusion leads to functional and histological protection when measured at 24 h; however, if measured at 48 h, protection is no longer evident. These studies suggest that caution should be exercised in extrapolating from the results of protective maneuvers in ischemic acute renal failure investigated by short-term studies.