The aim of this study was toinvestigate the effects of soluble amyloid βprotein (Aβ), with the amino acid sequence 25–35 (Aβ25–35), on delayed rectifier potassium current (Ik) in CA3 neurons of rat hippocampal slices using blind whole-cell patch-clamp recording techniques. We also studied the neurotoxic mechanism of soluble Aβ25–35 in Alzheimer’s disease. Results showed thatsoluble Aβ25–35 significantly suppressed Ik in CA3 neurons of rat hippocampal slices in time-dependent and voltage-dependent manners. Soluble Aβ25–35 significantly shifted the steady-state activation curve of Ik to more negative potentials. Before and after application of Aβ25–35, the half-activation potentials of Ik were 5.88 ± 0.67 and –2.81 ± 0.76 mV, respectively (n = 8; p < 0.05), but the slope factor was not significantly changed. The results suggest that the inhibitory effect of soluble Aβ25–35 on Ik may be one of the mechanisms of neurotoxicity, which may play an important role in the pathogenesis of Alzheimer’s disease.