Oleic acid (OA) uptake by rat adipocytes and the proportions of intracellular unesterified [3H]OA and its 3H-labeled esters were determined over 300 s. Uptake was linear for 20;-30 s, with rapid esterification indicating entry into normal metabolic pathways. Initial rates of OA uptake and its binding to plasma membranes were studied over a spectrum of oleic acid:bovine serum albumin (BSA) ratios, and expressed as functions of unbound OA concentrations calculated with both the 1971 OA:BSA association constants of Spector, Fletcher, and Ashbrook and more recent constants (e.g., the 1993 constants of Richieri, Anel, and Kleinfeld), which generate concentrations 10- to 100-fold lower. In either case, uptake was the sum of saturable and linear processes, with > or =90% occurring via the saturable pathway when the OA:BSA molar ratio was within the physiologic range (0.5;-3.0). Within this range, rate constants for saturable transmembrane influx (k(s)), calculated from both sets of constants, were similar (2.9 s(-1)) and were 10- to 30-fold faster than those for nonsaturable uptake (k(ns) = 0.26;-0.10 s(-1), t1/2 = 2.7;-6.6 s, based on the constants of Spector et al. and Richieri et al., respectively). The rate of oleic acid flip-flop into rat adipocytes (k(ff) = 0.16 +/- 0.02 s(-1), t1/2 = 4.3 +/- 0.5 s), computed from published data, was similar to k(ns). Thus, OA uptake occurs by both a saturable mechanism and passive flip-flop. This conclusion is independent of the OA:BSA association constants used to analyze the experimental measurements.