Omental fat cells were 30% smaller than those in subcutaneous regions. In omental fat cells with a mean diameter of 95 mu, the basal cAMP concentration was 50% lower, but the basal rate of glycerol release was three times as rapid as in subcutaneous (epigastric) fat cells of identical size. Added at maximal effective concentration, noradrenaline increased the level of cAMP and the rate of glycerol release more markedly in the omental than in the subcutaneous adipocytes, whereas the response to isopropyl noradrenaline was similar. Before starvation the lipolytic effects of noradrenaline and isopropyl noradrenaline, respectively, were identical in the two regions of subcutaneous adipose tissue investigated (femoral and hypogastric). The findings were well related to the tissue levels of cAMP induced by the two agents. During starvation noradrenaline and isopropyl noradrenaline increased the cAMP level and the rate of lipolysis in fat cells obtained from the hypogastric region, whereas noradrenaline decreased these parameters in femoral adipocytes. Starvation was associated with a more prominent inhibitory effect of phenylephrine on basal and isopropyl-noradrenaline-induced lipolysis in femoral than in hypogastric adipose tissue. In conclusion, differences exist between different regions of adipose tissue in their lipolytic responsiveness to noradrenaline, which seems related to the balance between alpha- and beta-adrenergic receptor response.