One of the essential roles of the human placenta is to produce the steroid hormone progesterone, which is required for the maintenance of pregnancy. The rate-determining step of placental progesterone synthesis is the conversion of cholesterol to pregnenolone by cytochrome P450scc (CYP11A1) in placental mitochondria in a reaction requiring electrons delivered via adrenodoxin reductase and adrenodoxin. Pregnenolone is converted to progesterone by type I 3 beta-hydroxysteroid dehydrogenase located in the mitochondrion. Progesterone synthesis by the human placenta displays notable differences from steroid synthesis in the classical steroid producing tissues such as the adrenal cortex and corpus luteum. One important difference is that the placenta lacks short term modulation of steroid synthesis and does not express the steroidogenic acute regulatory (StAR) protein. The most notable difference between the placenta and other steroidogenic tissues is that electron supply to P450scc limits the rate at which cholesterol is converted to pregnenolone in the placenta. The limiting component for electron delivery to P450scc is the concentration of adrenodoxin reductase in the mitochondrial matrix which is insufficient to maintain the adrenodoxin pool in a fully reduced state. Evidence suggests that placental mitochondria have a near-saturating cholesterol concentration for P450scc, likely provided by the StAR-like protein MLN64, and cholesterol translocation to the P450scc is not a major site of regulation of progesterone synthesis. Cyclic AMP stimulates progesterone synthesis by the human placenta but uncertainty remains regarding the key hormones that control cyclic AMP levels. The mechanism of regulation of adrenodoxin reductase levels in the human placenta remains to be studied. Placenta (2005), 26, 273-281 (c) 2004 Elsevier Ltd. All rights reserved.