Release of mitochondrial calcium has been shown to occur concomitant with mineral ion loading of matrix vesicles at the onset of mineralization in epiphyseal growth plate cartilage. Matrix vesicles contain amorphous calcium phosphate (ACP), a mineral form that usually results from rapid precipitation at high initial levels of Ca2+ and/or inorganic P (Pi). Since the cytosol of growth plate chondrocytes has been found to contain high levels of Pi, rapid release of mitochondrial Ca2+ into the cytosol may cause local precipitation of calcium phosphate and thus be coupled with matrix vesicle formation. Studies were carried out to determine the kinetics and nature of mineral formation that occur when small amounts of Ca2+ are added under various conditions to a Pi buffer composed of electrolytes matched in concentrations and pH to that of the cytosol of epiphyseal chondrocytes. Depending on the manner in which Ca2+ was added, ACP, dicalcium phosphate dihydrate (DCPD), or apatite (HA) first formed. In the presence of ATP, ACP was the only solid phase detected, being stable for at least 24 h. However, in its absence, ACP rapidly transformed into DCPD. Increasing the pH of the reaction buffer from 6.9 to 7.5 increased the amount of ACP initially formed, but DCPD was consistently found upon ACP transformation. Yet at pH 8.0, ACP persisted for at least 24 h. The amount of precipitate formed was proportional to the level of added Ca2+; precipitates formed when as little as 1.0 mmole was added per liter of buffer. Our findings support the possibility that rapid release of mitochondrial Ca2+ may cause localized intracellular precipitation of ACP. Since nascent ACP is known to stimulate membrane fusion and blebbing of vesicles, these findings may explain the presence of ACP in matrix vesicles. The rapid conversion of ACP to DCPD in the absence of ATP under these conditions may also explain the reported occurrence of DCPD in samples of early mineralizing tissue.