We have used whole-cell patch-clamp techniques to study the conductances in the plasma membranes of human parathyroid cells. With a KCl-rich pipette solution containing Ca2+ buffered to a concentration of 0.1 mumol/l, the zero current potential was -71.1 +/- 0.5 mV (n = 19) and the whole-cell current/voltage (I/V) relation had an inwardly rectifying and an outwardly rectifying component. The inwardly rectifying current activated instantaneously on hyperpolarization of the plasma membrane to potentials more negative than -80 mV, and a semi-logarithmic plot of the reversal potential of the inward current (estimated by extrapolation from the range in which it was linear) as a function of extracellular K+ concentration ([K+]o) revealed a linear relation with a slope of 64 mV per decade change in [K+]o, which is not significantly different from the Nernstian slope, demonstrating that the current was carried by K+ ions. The conductance exhibited a square root dependence on [K+]o as has been observed for inward rectifiers in other tissues. The current was blocked by the presence of Ba2+ (1 mmol/l) or Cs+ (1.5 mmol/l) in the bath. The outwardly rectifying current was activated by depolarization of the membrane potential to potentials more positive than -20 mV. It was inhibited by replacement of pipette K+ with Cs+, indicating that it also was a K+ current: it was partially (42%) blocked when tetraethylammonium (TEA+, 10 mmol/l) was added to the bath.(ABSTRACT TRUNCATED AT 250 WORDS)