Distribution and bioavailability of ⁴⁷calcium in milk labeled by extrinsic and intrinsic methods was investigated. Milk from Sprague Dawley rats was labeled by both methods, and milk from a cow was labeled by the extrinsic method. Retention of ⁴⁷Ca from milks administered to young male Sprague Dawley rats was determined through whole body counting for 6 days after administration of milk. Percent of ⁴⁷Ca dose retained was 72% for extrinsically labeled cow milk, 62% for extrinsically labeled rat milk, and 55% for intrinsically labeled rat milk. Samples were fractionated by ultracentrifugation and by gel exclusion chromatography. ⁴⁷Calcium distributions in rat milk labeled intrinsically or extrinsically were similar. The majority of ⁴⁷Ca was found in a particulate, > 30,000 molecular weight fraction (about 60% for cow milk, about 90% rat milks). The amount of milk calcium retained by rats appeared to be related to the amount of noncasein micelle-associated calcium. When administered by intraperitoneal injection into rats, ⁴⁵Ca specific activity of milk peaked in 60 to 90 minutes. Specific activity was highest in cytosol, and lower in Golgi apparatus and rough endoplasmic reticulum. Specific activities in subcellular fractions changed in parallel with specific activities of milk. Rapid turnover of Ca was observed in endoplasmic reticulum and Golgi apparatus; this was expected since secretory proteins and associated Ca are transported through these organelles for secretion. In vitro ⁴⁵Ca accumulation was compared in Golgi apparatus and endoplasmic reticulum from liver and mammary gland of lactating Dunkin Hartley guinea pigs. In the presence of ATP, highest accumulation per unit total fraction protein was found in Golgi apparatus (mammary gland 28% of available ⁴⁵Ca, liver 11%) while 8% was accumulated by endoplasmic reticulum fractions. Calcium accumulation was not the result of binding, as preincubation of vesicles with calcium ionophore resulted in less than 10% of the accumulation found without ionophore. The ATPase inhibitor sodium orthovanadate, and the ATP analog AMP-PNP, reduced ⁴⁵Ca accumulation in all fractions. Protonophore caused a small reduction in ⁴⁵Ca accumulation in all cases. Citrate accumulation by fractions was not observed under conditions used for ⁴⁵Ca accumulation. / Ph. D.