To determine the role of insulin and free fatty acid (FFA) concentration in the regulation of FFA metabolism, forearm FFA fluxes were quantified in 16 healthy volunteers by combining the forearm perfusion technique with the infusion of [3H]palmitate. Three groups of studies were performed. In study 1 (n = 6), a systemic insulin infusion (1.2 mU/kg.min) was performed for 120 min while euglycemia was maintained by a variable glucose infusion. In Study 2 (n = 5), insulin (0.05 mU/kg.min) was infused into the brachial artery to expose the forearm tissues to the same insulin level as in study 1. In study 3 (n = 5), heparin was infused to raise plasma FFA concentration to 1-1.5 mmol/L. At 60 min, an intrabrachial insulin infusion was added as in study 2 and maintained for 60 min. During systemic insulin infusion, plasma FFA concentration fell to 0.09 +/- 0.02 mmol/L. Forearm FFA uptake (FFA-U) decreased from the basal value of 2.54 +/- 0.52 to 0.95 +/- 0.10 mumol/L.min (P < 0.05). Likewise, forearm FFA release (FFA-R) fell to 1.0 +/- 0.31 mumol/L.min (P < 0.05). With local insulin administration, both FFA levels and FFA-U remained unchanged, whereas FFA-R was markedly inhibited (from 1.78 +/- 0.23 to 1.04 +/- 0.24 mumol/L.min; P < 0.05). In study 3 (heparin infusion), FFA levels rose to 1.17 +/- 0.12 mmol/L due to a 4-fold increase in FFA-R (from 1.18 +/- 0.36 to 6.92 +/- 2.40 mumol/L.min; P < 0.05). FFA-U rose from the basal value of 2.50 +/- 0.82 to 6.92 +/- 1.95 mumol/L.min (P < 0.05). Addition of intrabrachial insulin did not modify FFA-U, whereas heparin activation of FFA-R was only partially antagonized (4.53 +/- 2.40 mumol/L.min; 0.01 < P < 0.05 vs. heparin alone). The data demonstrate that plasma FFA concentration is the main determinant of forearm FFA transport. Insulin exerts a direct inhibitory effect on FFA release and affects tissue FFA transport only indirectly through the fall in circulating FFA.