The effects of Cd2+ on the hyperpolarization-activated K+-mediated current called IAB (Araque, A., and W. Bufio. 1994. Journal of Neuroscience. 14:399-408.) were studied under two-electrode voltage-clamp in opener muscle fibers of the crayfish Procambarus clarkii, IAB was reversibly reduced by extracellular Cd2+ in a concentration-dependent manner, obeying the Hill equation with IC5o = 0.452 ± 0.045 mM and a Hill coefficient of 1 (determined from the maximal chord conductance of IAB). Cd2+ decreased the IAB conductance (GAB) and shifted its voltage dependence towards hyperpolarized potentials in a similar degree, without affecting the slope of the voltage dependence. The IAB activation time constant increased, whereas the IAB deactivation time constant was not modified by Cd2+. The IAB equilibrium potential (EAB) was unmodified by Cd2+, indicating that the selective permeability of IAB channels was not altered. IAB was unaffected by intracellular Cd2+ The Cd2+ regulation of IAB did not depend on [K+]0, and the effects of [K+]0 on IAB were unchanged by Cd2+ indicating that Cd2+ did not compete with K+. Therefore, Cd2+ probably bound to a different site to that involved in the K+ permeability pathway. We conclude that Cd2+ affected the gating of IAB channels, interfering with their opening but not with their closing mechanism. The results can be explained by a kinetic model in which the binding of Cd2+ to the IAB channels would stabilize the gating apparatus at its resting position, increasing the energy barrier for the transition from the closed to the open channel states.