Concentrations of calcium, phosphorus, and 1,25-dihydroxyvitamin D in plasma of four young, four nonparetic aged, and four paretic aged cows were measured during the lactation cycle. Concentration of 1,25-dihydroxyvitamin D in plasma was elevated significantly in paretic aged cows as compared to nonparetic aged and young cows from the day of calving to 3 days postcalving. In paretic aged cows, severe hypocalcemia and hypophosphatemia developed on the day of calving, whereas hypocalcemia and hypophosphatemia were only transient in nonparetic aged and young cows at this time. Changes were only minor in concentration of 1,25-dihydroxyvitamin D in plasma from 7 days postcalving to 7 days precalving in all cows.