A membrane transport system for long chain fatty acids (FA) has been identified in the rat adipocyte. The process is freely reversible, does not require metabolic energy and has other characteristics of facilitated diffusion. FA transport can be stimulated up to 15 fold by catecholamines and ACTH. Insulin can completely suppress the stimulatory effect of catecholamine on FA transport but is without effect on the basal rate of uptake. Experiments suggest that accumulation of intracellular FA in the combined absence of glucose and the lipolytic inhibitor RHC80267 desensitizes the catecholamine-stimulated FA transport to insulin. In summary, the rate of FA transport at the level of the plasma membrane appears to play an important role in the control of FA mobilization by lipolytic and antilipolytic hormones.