Human periodontal ligament (PDL) fibroblasts expressed following two-pore-domain K(+) channels, TWIK-2 > TREK-1 > TWIK-1 >> TASK-1 > TRAAK > TASK-2. TREK-2 message was not detectable. We found the presence of arachidonic acid-activated and mechanical stress-sensitive K(+) channel, TREK-1, in the PDL fibroblasts by patch-clamp technique. It was also found the significant increase of intracellular concentration of arachidonic acid upon the application of cyclic stretch. Therefore, we suppose that the mechanical stretch due to the mastication activates phospholipase A(2) to release arachidonic acid (AA) from membrane, then, the released AA activates TREK-1. Thus, TREK-1 K(+) channels may play a protective role to maintain the negative membrane potential of PDL fibroblasts against the environmental stimuli.