In cultured human and rat cells, the lipolysis-stimulated receptor (LSR), when activated by free fatty acids (FFA), mediates the binding of apoprotein B- and apoprotein E-containing lipoproteins and their subsequent internalization and degradation. To better understand the physiological role of LSR, we developed a biochemical assay that optimizes both the activation and binding steps and, thus, allows the estimation of the number of LSR binding sites expressed in the livers of living animals. With this technique, a strong inverse correlation was found in rats between the apparent number of LSR binding sites in liver and the postprandial plasma triglyceride concentration (r = -0.828, p < 0.001, n = 12). No correlation existed between the number of LSR and plasma triglycerides measured in the same animals after 24 h of fasting. The same membrane binding assay was used to elucidate the mechanism by which FFA induce lipoprotein binding to LSR. The LSR activation step was mediated by direct interaction of FFA with LSR candidate proteins of apparent molecular masses of 115 and 90 kDa and occurred independently of the membrane lipid environment. The FFA-induced conformational shift that revealed the lipoprotein binding site remained fully reversible upon removal of the FFA. However, occupancy of the site by the apoprotein ligand stabilized the active form of LSR. Comparison of the effect of different FFA alone or in combination indicated that the same binding site is revealed by different FFA and that the length and saturation of the FFA monomeric carbon chain are critical in determining the potency of the FFA activating effect. We propose that the LSR pathway represents a limiting step for the cellular uptake of intestinally derived triglyceride-rich lipoproteins and speculate that FFA liberated by lipolysis initiate this process by altering the conformation of LSR to reveal the lipoprotein binding site.