Essentially pure (>95%) cultures of microglia were established from neopallia of newborn rats and used for whole-cell patch-clamp recording of electrophysiological properties and for proliferation studies. Two types of cultures were examined: 1) "Primary" cultures were grown in culture medium with serum and used within 3 weeks of isolation; 2) and "Colony-stimulating factor (CSF)-1-stimulated" cultures were derived from 3-week-old "primary" cultures by passaging and culturing them for several weeks longer in the presence of conditioned medium enriched in CSF-1. Microglia in the "primary" cultures expressed: 1) an inwardly rectifying K+ current (Kir) that was inhibited by Ba2+; 2) an outwardly rectifying K+ current (Kv) with many similarities to the cloned Kv1.3 channel of lymphocytes, including block by nanomolar concentrations of charybdotoxin (ChTX) and margatoxin (MgTX); and 3) an outwardly rectifying anion current with time- and voltage-independent gating. The anion current is activated reversibly under cell swelling conditions, i.e., after exposure to a hypo-osmotic bathing medium. The anion channels are highly permeable to Cl-, measurably permeable to gluconate (P(gluconate)/ PCl = 0.34), and blocked by flufenamic acid, 4-nitro-2-(3-phenylpropylamino)- benzoic acid (NPPB), and 6, 7-dichloro-2-cyclopentyl-2, 3-dihydro-2-methyl-1-oxo-1H-inden-5-yl (oxy) acetic acid (IAA-94). Microglia in the "CSF-1-stimulated" cultures expressed Kir and Cl- current, but not Kv current. Proliferation in the latter type of cultures could be slowed by omission of the CSF-1 enriched supernatant for 2 days and stimulated by adding back the conditioned medium. This "CSF-1-stimulated" proliferation was inhibited by Ba2+ (Kir blocker), and the Cl(-)-channel blockers flufenamic acid, NPPB, and IAA-94, whereas the Kv blockers ChTX and MgTX had no effect. Thus, Kir and Cl- channels appear to be necessary for "CSF-1-stimulated" proliferation of rat microglia, and there is no evidence that even a transient activation of Kv is necessary.