Fourteen healthy, lactating women (aged 25-38 y and between 2 and 8 mo postpartum) participated in both a maximal graded exercise test and a 30-min rest period to determine the influence of exercise on the concentration of selected milk minerals and electrolytes. Treatment order was randomized and treatments were conducted on different days. Milk was expressed before treatment and at 10, 30, and 60 min postexercise or after the rest period. Milk was wet-ashed by using nitric and sulfuric acids. Aliquots were analyzed for total phosphorus concentration by colorimetric assay and for calcium, magnesium, sodium, and potassium by inductively coupled plasma atomic emission spectrophotometry. Baseline mineral concentrations were not significantly different (P>0.05). Repeated expression of milk at 10, 30, and 60 min did not show significantly altered mineral concentrations (P>0.05). Interwoman variation was responsible for most of the variation in mineral concentrations. A portion of this may be explained by the variation in stage of lactation, which is known to affect mineral composition. Stage of lactation was inversely correlated with concentration of calcium, total phosphorus, magnesium, and sodium (P<0.05). In conclusion, maximal exercise did not alter concentrations of phosphorus, calcium, magnesium, potassium, or sodium in milk. Thus, with respect to mineral concentrations in milk, there is no contraindication for exercise during lactation.