Measurements of a frequency tunable magnetic metamaterial using metallic split rings loaded with barium strontium titanate thin film capacitors are presented. The resonant frequency of this medium is voltage tunable across a 140 MHz band centered at 1.75 GHz. S -parameter measurements in a microstrip waveguide reveal that the effective relative permeability of the slab has a roughly Lorentzian shape that reaches minimum values between -2 and -3 for biases from 0 to 5 V. The permeability of the slab can tune between positive and negative values, making it useful in applications requiring a state switchable magnetic permeability. © 2008 American Institute of Physics.