The lactate concentration (La) in breast milk has been shown to increase significantly following maximal (Int J Sports Med 12:328, 1991 & 12:395, 1992) and submaximal (J Womens Health 3: 91, 1994) exercise. It is not known how an increase in La in the milk will affect pH of the milk. The purpose of this study was to determine 1) if pH of breast milk changes with maximal exercise and 2) the relationship between milk La and pH. Eighteen lactating women (4.6±2.3 mo postpartum) participated in randomized control (30 min seated rest) and maximal exercise (VO2 max= 30.3±5.7 mL kg-1 min-1). Samples were collected at rest before and 10, 30 and 60 min after each treatment. La was measured by enzymatic techniques. ANOVA demonstrated significantly (*p<0.05) higher La (mM) at 10 and 30 min postexercise compared to rest and control, but no significant differences for pH. There was also no correlation between La and pH (r=0.053). Even though La increases in breast milk, pH of the milk is not altered. Thus, milk constituents appear to buffer pH changes resulting from exercise. Table