The orientation of 3-hydroxy-3-methylglutaryl coenzyme A reductase within the endoplasmic reticulum was investigated. Microsomal reductase activity was not latent, as addition of various detergents failed to activate the enzyme. Reductase activity was readily inhibited in intact microsomes by impermeable inhibitors such as trypsin, mercury dextran and anti-reductase IgG. Under the conditions used, these agents did not affect the intactness of microsomes as determined by latency of mannose-6-phosphate phosphohydrolase activity. The sensitivity to these inhibitors was not increased in disrupted microsomes. It is concluded that the domain containing the active site of the reductase is situated on the cytosolic surface of the endoplasmic reticulum.