Fetal rat bones were cultured in either growth-inducing or resorption-inducing media to study mineral losses during bone growth and atrophy in vitro. Whole radii and ulnae from 19-day-old fetal rats, prelabeled with 45Ca and/or 3H-tetracycline, were cultured intact or cut, and then digested by collagenase to obtain the calcified portion of the bones. Three- to five-fold more 3H-tetracycline than 45Ca was lost from the calcified portion when the bones were cultured for 4 days in growth-inducing media. Similar small amounts of 45Ca were lost from live and killed bones, but more 3H-tetracycline was lost from live bones than from killed bones. More 3H-tetracycline was released into the growth medium with a low concentration of calcium (0.5 mM) than when the calcium concentration was high (1.0 mM); no significant difference was seen in the release of 45Ca into the medium at different calcium concentrations. Larger amounts of both isotopes were lost when the prelabeled bones were cultured in resorption-inducing media than in growth-inducing media. When parathyroid hormone stimulated bone resorption in a resorption-inducing medium, equal proportions of both isotopes and bone collagen were lost. Greater losses of 3H-tetracycline than of 45Ca suggest that 45Ca was conserved locally during the resorption that accompanies bone growth, but not during resorption that accompanies bone atrophy.