The low frequency complex ac impedance Z of c-axis oriented YBa2Cu3O7 thin films was measured as a function of the applied magnetic field B. At temperatures not too close to the transition temperature the imaginary part of Z is proportional to B2, while the real part consists of a term proportional to B3 and an additional term proportional to exp(-1/B) which appears at higher fields. This behavior is in agreement with a description of vortex dynamics with a Langevin equation of motion.