The effects of stage of lactation and breed on total and ionized calcium in blood plasma were determined for eight Holstein and eight Jersey cows fed a diet containing .25% Ca during the dry period and fed a diet with .50% Ca after calving. Blood samples were obtained by jugular venipuncture at 2-wk intervals from 4 wk before calving through wk 10 of lactation. Total and ionized Ca contents of fresh plasma were determined potentiometrically with an ion-specific electrode. Total Ca in plasma also was determined by atomic absorption spectrophotometry. Average concentrations determined by atomic absorption were approximately .7 mg/dl higher than those for the ion-specific electrode. Plasma total and ionized Ca were lower in both breeds at calving than any other time during the sampling period. Jersey cows had lower total Ca (7.47 mg/dl) and ionized Ca (4.25 mg/dl) than Holsteins (8.10 and 4.66 mg/dl) on day of calving. Ionized Ca as a percentage of total calcium increased from 55 to 57% at parturition and then decreased slowly to 53% at peak lactation. Changes in percentage of ionized Ca may indicate the contribution of bone resorption relative to dietary Ca in meeting the Ca requirement for lactation.