1. The time- and voltage-dependent properties of a slowly inactivating and depolarization-activated potassium current and the functional consequences of its activation was investigated with current and single-electrode voltage-clamp techniques applied to guinea pig dorsal lateral geniculate neurons maintained as a slice in vitro. 2. In current clamp, application of a step depolarization to near firing threshold resulted in a slowly rising membrane potential that took up to 10 s to reach steady state and firing threshold. In voltage clamp, step depolarization of the membrane potential to values positive to approximately -65 mV resulted in the rapid activation followed by slow inactivation of an outward current. In both cases the sudden depolarization was associated with a large increase in membrane conductance, which gradually lessened in parallel with the slow depolarization in current clamp or with the decrease in outward current in voltage clamp. 3. The time course of inactivation of the outward current, which we refer to as IAs, was well fitted by a two-exponential function with time constants of 96 and 2,255 ms, suggesting the presence of a fast and slow phase of inactivation. The activation threshold for IAs was about -65 to -60 mV, whereas inactivation was incomplete even at -50 mV, suggesting the presence of a substantial "window" current. The time course of removal of inactivation of IAs at -85 to -100 mV was well fitted by a single exponential function with time constant of 91 ms. 4. IAs appears to be mediated by K+. Increasing [K+]o from 2.5 to 10 mM resulted in a reduction in amplitude of IAs, whereas changing from 10 to 2.5 mM [K+]o enhanced this current. Intracellular injection of Cs+ resulted in an abolition of IAs, whereas extracellular application of Ba2+ resulted in a large decrease in the apparent input conductance but relatively little reduction of IAs. 5. Both phases of inactivation of the transient outward current were completely blocked by low doses (100 microM) of 4-aminopyridine (4-AP), but not by extracellular application of Cs+, tetraethylammonium (TEA), tetrodotoxin (TTX), or after block of transmembrane Ca2+ currents. Local application of 4-AP to neurons depolarized to near firing threshold resulted in depolarization associated with a decrease in apparent input conductance, thereby confirming the presence of a window current.4+ this bias against depolarizing inputs.(ABSTRACT TRUNCATED AT 400 WORDS)