Although it has been demonstrated that during isotonic grip exercise patients with chronic congestive heart failure have an abnormally reduced forearm oxygen consumption resulting from a depressed forearm flood flow, there may be additional etiologies of this abnormality. To explore this possibility biopsies of the pronator teres muscle were taken in eight control subjects with normal cardiac hemodynamics and in seven severely decompensated chronic congestive heart failure subjects. Each sample was fixed, stained, and photomicrographs of the sections were obtained and the capillary basement membrane thickness determined. The control capillary basement membrane thickness was 3028 plus or minus 187 A (mean plus or minus SEM) compared to the congestive heart failure thickness of 4924 plus or minus 538 A (pless than .01). It is possible that the increased basement membrane thickness in congestive heart failure may result from or actually cause the depressed oxygen consumption by altering diffusion.