Drugs that influence the opening of potassium (K+) channels and as a consequence cause hyperpolarization of cell membrane possess clinical potential. The large conductance Ca2+-activated K+ (BK) channel is highly selective for K+. Activation of this channel is Ca2+- and voltage-dependent. We have investigated the effects of thymol, a natural product, on ion currents in pituitary GH3 cells. The patch-clamp technique was used to investigate the effect of thymol (100 μM) in these cells. Thymol reversibly stimulated the Ca2+-activated K+ current with an EC50 value of 75 pM. In a cell-attached configuration, application of thymol to the bath increased the activity of BK channels. BAPTA (1 mM) attenuated thymol-stimulated channel activity. In an experiment using the inside-out configuration, thymol exposed to the intracellular face of excised patches did not modify the single-channel conductance of these channels whereas it enhanced the channel activity. Neither menthol (100 μM) nor zingerone (100 μM) had an effect on BK-channel activity while AAPH (100 μM) suppressed it significantly. The stimulatory actions of thymol on Ca2+-activated K+ currents may be associated with the underlying cellular mechanisms through which it affects neuronal or neuroendocrine functions.