In adrenal zona glomerulosa cells, the calcium messenger system is the major signaling mechanism activated by physiological stimulators of aldosterone production. We present here evidence for a dual site of action of the calcium signal: 1) Calcium influx into the mitochondrion is a prerequisite to the activation of steroidogenesis. This calcium entry leads to a rise in mitochondrial calcium concentration and to an increase in intramitochondrial cholesterol transfer and Steroidogenic Acute Regulatory (StAR) protein accumulation in inner mitochondrial membranes. 2) Calcium also exerts a genomic regulatory effect by activating transcription of the StAR gene.