Patch-clamp recordings were made on rat hippocampal neurons maintained in culture. In cell-attached and excised inside-out and outside-out patches a large single-channel current was observed. This channel had a conductance of 220 and 100 pS in 140 mM [K+]i/140 mM [K+]o and 140 mM [K+]i/3 mM [K+]o respectively. From the reversal potential the channel was highly selective for K+, the PK+/PNa+ ratio being 50/1. Channel activity was voltage-dependent, the open probability at 100 nM [Ca2+]i increasing by e-fold for a 22 mV depolarization. It was also dependent on [Ca2+]i at both resting and depolarized membrane potentials. Channel open states were best described by the sum of two exponentials with time constants that increased as the membrane potential became more positive. Channel activity was sensitive to both external (500 microM) and internal (5 mM) tetraethylammonium chloride. These data are consistent with the properties of maxi-K+ channels described in other preparations, and further suggest a role for maxi-channel activity in regulating neuronal excitability at the resting membrane potential. Channel activity was not altered by 8-chlorophenyl thio cAMP, concanavalin A, pH reduction or neuraminidase. In two of five patches lemakalim (BRL 38227) increased channel activity. Internal ruthenium red (10 microM) blocked the channel by shortening the duration of both open states. This change in channel gating was distinct from the 'mode switching' seen in two patches, where a channel switched spontaneously from normal activity typified by two open states to a mode where only short openings were represented.