1. Human leucocyte sodium pump activity was studied in normal fasting subjects by measuring the ouabain-sensitive 22Na+ efflux rate constants. 2. This 22Na+ efflux rate constant was inversely related to the fasting plasma non-esterified fatty acid level (rs = −0.73, P < 0.0001). 3. An oral glucose load (40 g/m2 surface area) led to an increase in the leucocyte ouabain-sensitive 22Na+ efflux rate constant after 2 h (1.97 ± 0.25 to 2.44 ± 0.19 h−1, P < 0.0001, n = 11). There was a concomitant fall in the plasma non-esterified fatty acid level. 4. Incubation of leucocytes in vitro with 100 μmol/l linoleic acid inhibited the leucocyte ouabain-sensitive 22Na+ efflux rate constant (1.52 ± 0.27 vs 0.84 ± 0.24 h−1, P < 0.001, n = 8). 5. The leucocyte Na+,K+-dependent adenosine triphosphatase (Na+,K+-ATPase) activity was inhibited in vitro by long chain non-esterified fatty acids, especially when unsaturated. 6. Non-esterified fatty acids may account for some of the Na+,K+-ATPase inhibitory activity of plasma.