Linoleic acid (LA) had two effects on human Kv1.5 and Kv2.1 channels expressed in Chinese hamster ovary cells: an increase in the speed of current activation process (EC₅₀ = 2.4 and 2.7 µM for Kv1.5 and Kv2.1, respectively) and current inhibition (IC₅₀ = 6.6 and 7.4 for Kv1.5 and Kv2.1, respectively). LA affected the activation kinetics via two processes: a leftward shift in the instantaneous activation curves and an increase in the rate of current rise. Current inhibition by LA was time dependent but voltage independent. Hill slopes for plots of current inhibition (3.5 and 3.9 for Kv1.5 and Kv2.1, respectively) vs. dose of LA suggested that cooperativity was involved in the mechanism of current inhibition. A similar analysis of the effects of LA on current activation did not reveal cooperative interactions. The effects of LA were mediated from the external side of the channels, since addition of 10 µM LA to the patch pipette solution was without effect. Additionally, the methyl ester of LA was effective at enhancing peak current and promoting channel activation for Kv1.5 and Kv2.1 without inducing significant current inhibition.