1. Potassium currents have been recorded from rat hippocampal neurons in dissociated cultures prepared at E17-E19. Currents were studied with the whole-cell version of the patch clamp method. The kinetics and pharmacological properties of two transient outward currents have been characterized. 2. Most of the recordings have been done in cells which had been in culture 10-18 days. Both a fast and a slow transient current could be elicited. A subtraction procedure was used to isolate the fast transient current. The fast transient current decayed monoexponentially with a time constant of about 10 ms. The slow transient current decayed with two time constants in the order of 500 ms and of 3.4 s. The reversal potential of the slow current shifted by 54 mV for a tenfold change in extracellular potassium concentration. 3. Studies on the removal of inactivation for the two currents revealed time constants of 29 and 107 ms for the fast and slow transient current, respectively. 4. The steady-state inactivation properties of the fast transient current were determined by studying the current with a fixed depolarizing command of -10 mV and varying pre-pulse amplitudes from a holding potential of -50 mV. The inactivation curve could be fitted with a Boltzmann equation. Half-maximal inactivation occurred at -81 mV. The steady-state activation properties of the fast transient current were determined by varying the depolarizing voltage commands following a fixed pre-pulse to -110 mV. The threshold for activation was between -70 and -60 mV. Half-maximal activation was reached at -19 mV. 5. The steady-state inactivation properties of the slow transient current were determined by studying the current elicited by varying the hyperpolarizing voltage steps from a holding potential of 0 mV. The inactivation curve could be fitted with a Boltzmann equation. Half-maximal inactivation was obtained at -61 mV. The steady-state activation properties were determined in a manner similar to the fast current. The threshold for activation was between -40 and -30 mV. 6. The slow transient current was not inactivated immediately when the conditioning pre-pulse was stopped. The rate of current decay increased with stimulus frequency. 7. Both transient currents were sensitive to 4-aminopyridine (4-AP). The fast transient current was blocked completely by 5 mM provided a pre-pulse of 1 s to -110 mV was employed. The slow transient current was already depressed by 4-AP applied in the 100 microM range but could never be blocked completely.(ABSTRACT TRUNCATED AT 400 WORDS)