The ionic current underlying anomalous rectification in opener muscle fibers of crayfish was studied under two-electrode voltage clamp. Opener muscle fibers showed a mean resting potential (RP) of -64.8 mV and an input resistance of 0.4 M omega. Hyperpolarizing voltage command pulses from a holding potential (H) of -60 mV evoked an instantaneous voltage-independent linear current (IL) followed by a time- and voltage- dependent inward current (IAB) that reached a steady state within 500 msec. The reversal potential of IAB (EAB) was estimated from tail current amplitudes. At an extracellular K+ concentration ([K+]o) of 5.4 mm the mean EAB was -61.8 mV. EAB shifted toward positive potentials by 50.8 mV for a 10-fold increase in [K+]o. The conductance underlying IAB (GAB) increased sigmoidally with hyperpolarization, starting close to the RP, saturating at a GAB,max of about -140 mV, and showing a mean half-activation at -94.4 mV. The activation curve of GAB shifted 53.6 mV toward positive potentials with a 10-fold increase in [K+]o. GAB,max did not increase in raised [K+]o. The activation and deactivation kinetics of IAB were accurately described by single exponentials with similar time constants (< 100 msec). Time constants changed as an exponential function of the membrane potential. IAB, its time course, GAB, and EAB were not modified in the following conditions: (1) Na(+)- and Ca(2+)-free solutions, (2) intracellular EGTA, (3) extracellular (100 mM) or intracellular tetraethylammonium, (4) extracellular Cs+ (up to 50 mM), Rb+ (up to 10 mM), Ba2+ (13.5 mM), or Mn2+ (13.5 mM). However, low extracellular concentrations of Cd2+ or Zn2+ strongly and reversibly reduced both IL and IAB. Therefore, we conclude that anomalous rectification in crayfish muscle is generated by a voltage- and time-dependent K+ current IAB. This current displayed many electrophysiological and pharmacological characteristics that distinguished it from all others mediating anomalous rectification described previously.