Adipose tissue (AT) is more than a collection of adipocytes. It is a highly organized tissue in which different cell types interact, and in which a complex mix of hormones and substrates arriving in the plasma, together with neural input and the rate of blood flow (BF), all regulate metabolic activity. These multiple, interacting influences cannot be reproduced in vitro, and, hence, if we wish to understand the integration of AT metabolism in the whole body, it is essential to perform studies of AT metabolism in vivo. There are a number of ways in which this can be done (1). One of the most specific and informative, in a quantitative sense, is the measurement of arteriovenous (A-V) differences across the tissue.