1. Inward and outward currents were recorded in the human Jurkat T cell line using the whole-cell configuration of the patch-clamp technique. 2. The transient outward current was activated at membrane potentials positive to -60 mV. The activation time constant-voltage relationship decreased from 17 ms to 2 ms for membrane potentials ranging from -40 to +40 mV. The inactivation phase could be fitted by a single-exponential function and the inactivation time constant decreased from 250 ms to 150 ms for membrane potentials ranging from -20 to +100 mV. 3. The steady-state inactivation-voltage relationship showed a mid-point potential of -32 +/- 2.6 mV, and the slope factor was 10.8 +/- 1.8 mv (n = 3). 4. The calcium ionophore A23187 provoked a decrease in the amplitude of the outward current, suggesting a dependence of this current on the cytosolic concentration of Ca2+. 5. The K+ outward current was blocked by tetraethylammonium (TEA, Michaelis-Menten constant (Km), 6 mM) and by the calcium channel blockers Ni2+, Co2+, Mn2+ and Cd2+. 6. Forty per cent (n = 120) of the patched Jurkat cells displayed an inward current. In a physiological medium containing Ca2+ (2.2 mM), the inward current threshold voltage was -60 mV, the maximum current was observed at -40 mV and the zero current voltage was positive to +20 mV. At negative membrane potentials, the time required to reach 50% of the maximum amplitude was 60 ms and grew shorter with increasing depolarization, reaching a value of 5 ms at -5 mV. The inactivation of the inward current was very slow and the time constant varied from 1200 ms at -35 mV to approximately 250 ms for potentials positive to -10 mV. 7. The current availability had a value of one for potentials negative to -50 mV and zero for potentials positive to -15 mV. The mid-point potential was -31 +/- 3.4 mV and the slope factor was 3.3 +/- 0.2 mV (n = 3). 8. The inward channels were permeable to Sr2+, but were blocked by classical Ca2+ channel inhibitors such as Co2+, Mn2+ and Ni2+. 9. Phaseolus vulgaris phytohaemagglutinin (PHA), an inducer of interleukin-2 production in Jurkat cells, increased the inward current amplitude by 32 +/- 20% (n = 4). This increase was concomitant with a decrease (45 +/- 12%) in the amplitude of the outward current, but only when the current was carried by Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)