The effects of histamine on the membrane potential and currents of human glioblastoma (GL-15) cells were investigated. In perforated whole-cell configuration, short (3 s) applications of histamine (100 µM) hyperpolarized the membrane by activating a K⁺-selective current. The response involved the activation of the pyrilamine-sensitive H₁ receptor, and Ca²⁺ release from thapsigargin-sensitive intracellular stores. The histamine-activated current was insensitive to TEA (3 mM), iberiotoxin (100 nM), and d-tubocurarine (100 µM), but markedly inhibited by charybdotoxin (100 nM), clotrimazole (1 µM) and TRAM-34 (1 µM), a pharmacological profile congruent with the intermediate-conductance Ca²⁺-activated K⁺ (IK_Ca) channel. Cell-attached recordings confirmed that histamine activated a K⁺ channel with properties congruent with the IK_Ca channel (voltage-independence, 22 pS unitary conductance and slight inward rectification in symmetrical 140 mM K⁺). More prolonged histamine applications (2-3 min) often evoked a sustained IK_Ca channel activity, that depended on a La²⁺ (10 µM)-sensitive Ca²⁺ influx. Intracellular Ca²⁺ measurements revealed that the sustained IK_Ca channel activity enhanced the histamine-induced Ca²⁺ signal, most likely by a hyperpolarization-induced increase in the driving force for Ca²⁺ influx. In virtually all cells examined we also observed the expression of the large-conductance Ca²⁺-activated K⁺ (BKCa) channel, with a unitary conductance of ca. 230 pS in symmetrical 140 mM K⁺, and a KD(Ca) of ca. 3 µM, at -40 mV. Notably in no instance was the BK_Ca channel activated by histamine under resting conditions. The most parsimonious explanation based on the different K_D(Ca) for the two K_Ca channels is provided.