[en] The activation of mitochondrial Ca²⁺ uptake by Ca²⁺, and by the endogenous polyamines, spermine and spermidine, was investigated in regard to both the mechanism of activation, and to the developmental changes and brain regional differences of these effects. Ca²⁺ uptake by isolated brain mitochondria was characterized by analysis of ⁴⁵Ca²⁺ uptake and by use of a Ca²⁺-selective electrode. A novel Ca²⁺ loading procedure was used to estimate steady-state rates of uptake, and the results were compared to the initial rates of ⁴⁵Ca²⁺ uptake