The initiation of lactation involves an increased flow of Ca into mammary secretions, which leads to responses of serum concentrations of Ca and parathyroid hormone (PTH) that may be influenced by dietary Ca. Eight light mares from Farm A and eight Thoroughbred mares from Farm B were bled and milked 10 d pre-foaling, and eight mares (four from each farm) were bled and milked 5 d post-foaling. Milk Ca was measured by two commercial tests, one for [Ca + Mg] and the other [Ca]. Serum PTH and total Ca were measured in 16 mares, and ionized Ca in four mares. Parturition was induced in all mares with fenoprostalene on Farm A, and in four mares with oxytocin on Farm B; no significant difference was found between induction methods or between induced and spontaneous foaling mares. Dietary Ca was 34% DM on Farm A and .79% on Farm B. Mean serum total Ca concentrations decreased from 12.5 mg/dl to a nadir of 11 mg/dl on d 2 post-partum, and mean PTH increased from 46 pg/ml to a peak of 186 pg/ml on d 2 post-partum. Mean serum PTH concentrations were lower (P = .03) and total Ca concentrations were higher (P = -01) on Farm B in comparison to Farm A, probably reflecting the difference in Ca intake. The nadir in mean ionized Ca and total Ca concentrations was reached on d 2 post-partum, 1 day later than has been observed previously in the dairy cow. Milk Ca concentrations increased from 50 ppm 7 d pre-foaling to 350 ppm on the day of foaling, with no difference between farms. The [Ca + Mg] test reached a critical level of 200 ppm 4.5 d pre-foaling, the [Ca] test 2 d pre-foaling. The [Ca + Mg] and [Ca] tests reached 250 ppm 2.5 and 1 d pre-partum, respectively. In short, serum Ca and PTH concentrations showed periparturient changes which reflected dietary Ca pre-partum. Foaling date was more closely associated with milk [Ca] than with [Ca + Mg] and by a critical level of 250 ppm than by 200 ppm. / Master of Science