The magnetoimpedance spectra (0 less than or equal to f less than or equal to 3 MHz) of amorphous and of nanocrystallized (550 degrees C annealing for 0.5-18 h) FeCuNbSiB ribbons were measured at room temperature using a commercial impedance analyzer as a function of applied magnetic field up to 75 Oe. The magnetoimpedance (MI), Z(f, H), shows strong dependence on frequency of the drive, ac current and magnetic field. For all the samples, the MI ratio Delta Z/Z = (Z(H-max - Z(0)\Z(H-max) first increases rapidly with frequency (f) and reaches a maximum at about 300-500 kHz: then decreases with f. The maximum value of the MI ratio Delta Z/Z, - 227%, is obtained in the sample annealed at 550 degrees C for 3 h with ac current at 300 kHz. At higher frequency, apeak in field dependence of the MI ratio (Z(H) - Z(0))/Z(H-max) was obtained for nanocrystalline samples, but not for as-quenched amorphous counterparts. The field dependence of effective permeability has been shown to be responsible for their MI effects. (C) 1997 Elsevier Science S.A.