Sixteen male dogs had split renal function studies prior to unilateral nephrectomy and autotransplant. Kidneys were preserved for 24 hr by either simple hypothermic storage in Collins C2 solution (SHS) or machine-perfused (MP) on a Waters machine (MOX 100) with plasmanate perfusate. Renal function studies were repeated at 1 hr and at 7, 14, and 28 days, and the statistical relationship between initial and 1-month function was determined for a number of parameters. All MP kidneys functioned immediately, whereas 1/3 of SHS kidneys had delayed function. Recovery was more rapid in MP kidneys and was essentially complete by 14 days, at which time MP kidneys had higher rates of creatinine clearance and sodium reabsorption. However, by one month 3/7 MP kidneys (P = 0.15 compared with SHS) had lower creatinine clearance rates than at 2 weeks, and para-aminohippurate (PAH) clearance and fractional sodium reabsorption were significantly decreased. During the same period SHS kidneys either showed continued improvement or maintained stable function. Thus, by one month there were no differences between the groups in clearances of creatinine and PAH, plasma creatinine and blood urea nitrogen concentrations, or fractional reabsorption of sodium, potassium, and water. For SHS kidneys, the 1-hr creatinine clearance and the absolute rate of sodium reabsorption were strong predictors of the eventual function of the kidneys at one month (r = 0.93 and r = .83, P less than 0.05, respectively). No such correlations were found in MP kidneys (r = less than .01, P greater than 0.9 for both variables). The data show that MP results in significantly better function early after transplant, but this advantage does not persist, and that SHS kidneys early function is a good predictor of long-term recovery, but this is not true for MP kidneys.