Changes in mare mammary secretion composition were characterized prior to and after foaling. Eighteen mares were used for collection of mammary secretions from 23 days prepartum through 44 days postpartum. Concentrations of lactose, total protein, IgG and lysozyme (activity assay) were determined. Considerable variabilty was observed among mares. Mean lactose concentrations by mare were lower (P < .05) in the prepartum period compared with the postpartum period. Mean total protein and immunoglobulin G concentrations by mare were higher (P < .05) in the prepartum period compared with the postpartum period. Mean lysozyme activities by mare tended to be lower (P < .08) during the prepartum period compared with the postpartum period. Mammary secretion protein profiles, characterized by SDS-PAGE, were consistent with changes in concentrations of specific proteins measured in the secretions. Dynamic changes occurring in mammary secretions during the transition from prepartum accumulation of colostrum to postpartum production of milk include factors like IgG and lysozyme which have protective roles in the neonatal foal.