The flow of granular nickel particles moving down a vertical pipe from a hopper in the presence of a local, horizontal ac electric field is studied. It is found that the flow can be retarded by the electric field, and the retardation depends upon the initial conditions of the flow. If the flow is dilute when passing through the field, there exists a critical voltageV(1), the flow transition from dilute to dense occurs and flow rate is retarded when V is greater than V-1. For V < V-1, the steady-state flow rate is practically unaffected. The flow rate decreases abruptly at V-1. Particle aggregations near the top end of the electrodes are observed. A two-dimensional molecular dynamics (MD) simulation is performed. It confirms the aggregation and intermittence of the flow, and the abrupt change in flow rate at the dilute-dense transition. The MD simulation results agree qualitatively well with the experimental observations. (C) 2003 Elsevier B.V. All rights reserved.