Assessment of mammary gland secretion (MGS) pH is an inexpensive method to determine impending parturition in mares. However, previous studies have shown that some mares may fail to show changes in MGS before foaling. Following collection of MGS, it is unknown the ideal conditions for sample storage and time for assessment of pH. These questions are relevant in practice for optimal use of pH of MGS. Our objectives were (1) to determine MGS pH and electrolyte concentrations prepartum and at parturition, (3) to characterize milk pH in the first week postpartum, and (3) to evaluate pre-foaling MGS pH at 3 storage temperatures. We hypothesized that (1) MGS has an acidic pH, and electrolyte inversion at the time of foaling regardless of prepartum values, and (2) MGS pH varies with storage temperature and time. Healthy term light breed mares (n = 25) were examined daily, and small aliquots of MGS were collected twice a day until foaling. MGS pH was measured with a portable pH meter. Ca²⁺, Na⁺, K⁺ and Mg²⁺ concentrations were measured using an automated analyzer (AU 480 Beckman Coulter) for 7 d pre-foaling. Eighteen MGS aliquots were collected and equally divided into 3 storage conditions: 37°C, 21°C, and 5°C. pH was measured at 0, 15, 30, 45 and 60 min., and hourly for 10h. Postpartum milk samples were collected daily for 7 d to evaluate pH. All data analyses used JMP 12.1 (SAS Institute Cary, NC, USA). Electrolytes and pH were analyzed using mixed models and Tukey’s LSD test. Milk pH for the first week postpartum was compared with a mixed model. Significance set at P ≤ 0.05. All mares had high Ca²⁺, Mg²⁺, K⁺, low Na⁺, and an acidic pH at parturition. MGS remained slightly acidic (pH 6.8–6.9) for 2 d postpartum before increasing by d 3 (7–7.1). Storage temperature did not affect MGS pH for 45m of storage. All mares change MGS pH and electrolytes before foaling, but in a very small portion of mares change so rapidly that twice a day sampling may miss the change for foaling prediction.