Nucleotide‐induced currents in untreated (proliferating) and lipopolysaccharide (LPS; 100ngml−1) treated (non‐proliferating) rat microglial cells were recorded by the whole‐cell patch‐clamp technique. Most experiments were carried out on non‐proliferating microglial cells. ATP (100nm–1mm), ADP (10nm–10mm) and UTP (1μm–100mm), but not uridine (100μm–10mm) produced a slow outward current at a holding potential of 0mV. The effect of UTP (1mm) did not depend on the presence of extracellular Mg2+ (1mm). The outward current response to UTP (1mm) was similar in non‐proliferating and proliferating microglia. In non‐proliferating microglial cells, the ATP (10μm)‐induced outward current was antagonized by suramin (300μm) or reactive blue 2 (50μm), whereas 8‐(p‐sulphophenyl)‐theophylline (8‐SPT; 100μm) was inactive. By contrast, the current induced by UTP (1mm) was increased by suramin (300μm) and was not altered by reactive blue 2 (50μm) or 8‐SPT (100μm). The current response to UTP (1mm) disappeared when K+ was replaced in the pipette solution by an equimolar concentration of Cs+ (150mm). However, the effect of UTP (1mm) did not change when most Cl− was replaced with an equimolar concentration of gluconate− (145mm). The application of 4‐aminopyridine (1mm) or Cs+ (1mm) to the bath solution failed to alter the UTP (1mm)‐induced current. UTP (1mm) had almost no effect in a nominally Ca2+‐free bath medium, or in the presence of charybdotoxin (0.1μm); the inclusion of U‐73122 (5μm) or heparin (5mgml−1) into the pipette solution also blocked the responses to UTP (1mm). By contrast, the effect of ATP (10μm) persisted under these conditions. I‐V relations were determined by delivering fast voltage ramps before and during the application of UTP (1mm). In the presence of extracellular Cs+ (1mm) and 4‐aminopyridine (1mm) the UTP‐evoked current crossed the zero current level near−75mV. Omission of Ca2+ from the Cs+ (1mm)‐ and 4‐aminopyridine (1mm)‐containing bath medium or replacement of K+ by Cs+ (150mm) in the pipette solution abolished the UTP current. Replacement of GTP (200μm) by GDP‐β‐S (200μm) in the pipette solution abolished the current evoked by UTP (1mm). When the pipette solution contained Cs+ (150mm) instead of K+ and in addition inositol 1,4,5,‐trisphosphate (InsP3; 10μm), an inward current absolutely dependent on extracellular Ca2+ was activated after the establishment of whole‐cell recording conditions. This current had a typical delay, a rather slow time course and did not reverse its amplitude up to 100mV, as measured by fast voltage ramps. A rise of the internal free Ca2+ concentration from 0.01 to 0.5μm on excised inside‐out membrane patches produced single channel activity with a reversal potential of 0mV in a symmetrical K+ solution. The reversal potential was shifted to negative values, when the extracellular K+ concentration was decreased from 144 to 32mm. By contrast, a decrease of the extracellular Cl− concentration from 164 to 38mm did not change the reversal potential. Purine and pyrimidine nucleotides ...