(Supported in part by NIH HD-06274 to J. F. Strauss III) The purpose of this study is to determine whether the steroidogenic acute regulatory protein (STAR), affects the structure of mitochondria, particularly the relationship between the mitochondrial outer and inner membranes. STAR is known to play a key role in the regulation of the conversion of cholesterol into pregnenolone, a reaction that is catalyzed by an ezyme complex located on the inner mitochondrial membranes. The rate-limiting step in the reaction is the movement of cholesterol from the outer to the inner mitochondrial membranes. Our working hypothesis is that STAR promotes contact or fusion of mitochondrial outer and inner membranes allowing cholesterol to move down a chemical gradient to the cholesterol side-chain cleavage enzyme. COS-1 adrenal tissue-culture cells transduced with an adenoviral vector expressing STAR or a control protein, LacZ, have been processed for ultrastructural analysis. Two double-tilt topographic reconstructions were made from 0.25lm thick sections using the IVEM, one of a mitochondrion from a non-infected control cell, and one from a mitochondrion from a cell with the STAR protein. We are still analyzing the results, and need to do more reconstructions before we can draw any conclusions.