The purpose of this study was to investigate the concentration of lactic acid in breast milk following maximal exercise. Seven active postpartum women (2 to 24 months) were exercised to maximum on a treadmill (VO2 max = 37.0 +/- 13.7 ml.min-1.kg-1). Blood was sampled via finger prick at rest before exercise and 5 minutes postexercise; milk was collected via self expression at rest before exercise and at 10 and 30 minutes postexercise. Both blood and milk were analyzed for lactic acid via enzymatic methods. Following maximal exercise, a significant increase in the concentration of lactic acid was found in the blood at 5 minutes postexercise and breast milk at 10 minutes postexercise. Although elevated, the lactic acid concentration of the 30-minute sample of breast milk was not significantly different from the resting sample. Maximal exercise can result in significant increase in lactic acid concentration in breast milk. Further research is needed to demonstrate whether the taste of the milk is affected.