Mechanisms of proton conductance (GH) were investigated in phospholipid bilayer membranes containing long-chain fatty acids (lauric, myristic, palmitic, oleic or phytanic). Membranes were formed from diphytanoyl phosphatidylcholine in decane plus chlorodecane (usually 30% vol/vol). Fatty acids were added either to the aqueous phase or to the membrane-forming solution. Proton conductance was calculated from the steady-state total conductance and the H+ diffusion potential produced by a transmembrane pH gradient. Fatty acids caused GH to increase in proportion to the first power of the fatty acid concentration. The GH induced by fatty acids was inhibited by phloretin, low pH and serum albumin. GH was increased by chlorodecane, and the voltage dependence of GH was superlinear. The results suggest that fatty acids act as simple (A- type) proton carriers. The membrane: water partition coefficient (Kp) and adsorption coefficient (beta) were estimated by finding the membrane and aqueous fatty acid concentrations which gave identical values of GH. For palmitic and oleic acids Kp was about 10(5) and beta was about 10(-2) cm. The A- translocation or "flip-flop" rate (ka) was estimated from the value of GH and the fatty acid concentration in the membrane, assuming that A- translocation was the rate limiting step in H+ transport. The kA's were about 10(-4) sec-1, slower than classical weak-acid uncouplers by a factor of 10(5). Although long-chain fatty acids are relatively inefficient H+ carriers, they may cause significant biological H- conductance when present in the membrane at high concentrations, e.g., in ischemia, hypoxia, hormonally induced lipolysis, or certain hereditary disorders, e.g., Refsum's (phytanic acid storage) disease.