Autonomic nervous function evaluated by heart rate variability (HRV) and blood characteristics were compared between Holstein Friesian cows that developed postpartum fever (PF; n = 5) and clinically healthy (CH; n = 6) puerperal cows in this case-control study. A cow was defined as having PF when its rectal temperature rose to >/=39.5 degrees C between 1 and 3 days postpartum. We recorded electrocardiograms during this period using a Holter-type electrocardiograph and applied power spectral analysis of HRV. Comparisons between the groups were analyzed by t test or Mann-Whitney U test, and the relationship between rectal temperature and each parameter was analyzed using multiple regression analysis. Heart rate was higher in PF cows than in CH cows (Mean +/- SE, 103.3 +/- 2.7 vs. 91.5 +/- 1.7 bpm). This result suggested that PF cows had a relatively dominant sympathetic nervous function. Total (44,472 +/- 2,301 vs. 55,373 +/- 1,997 ms) and low frequency power (24.5 +/- 3.8 vs. 39.9 +/- 5.3 ms) were lower in PF cows than in CH cows. These findings were possibly caused by a reduction in autonomic nervous function. The total white blood cell count (54.3 +/- 5.1 vs. 84.5 +/- 6.4 x102/muL) and the serum magnesium (2.1 +/- 0.1 vs. 2.4 +/- 0.1 mg/dL) and iron (81.5 +/- 8.0 vs. 134.4 +/- 9.1 mug/dL) concentrations were lower and the serum amyloid A concentration (277 +/- 33 vs. 149 +/- 21 mug/mL) was higher in PF cows than in CH cows. These results imply that more inflammation was present in PF cows than in CH cows. Multiple regression analysis showed that both of low frequency power and concentration of serum iron were associated with rectal temperature. We found differences in changes in hematologic results, biochemical findings, and HRV patterns between PF cows and CH cows.