The effects of different ionic media on Ca2+ uptake and release in isolated brain microsomes were investigated. KCl (100 mM) provided the best medium for Ca2+ uptake in the presence of ATP. The effect of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ release was examined and was maximum at 0.2 microM. IP3-induced Ca2+ release was dependent on extramicrosomal free Ca2+ concentration with maximal release at 5.0 microM free Ca2+. Replacement of KCl by sucrose or NaCl did not show any response to IP3. Electron microscopy showed that the microsomal fraction consisted of characteristic endoplasmic reticulum-derived vesicular profiles and were free of mitochondria or plasma membrane contamination. Our results support the concept that the endoplasmic reticulum is the target for IP-3 induced mobilization of Ca2+ in the cell.