The energy dissipation in a magnetic fluid placed in AC magnetic field of the sound frequency is studied experimentally. The aim of the paper is to obtain information on the amplitude dependence of the dynamic susceptibility and the limits of applicability of the simplest (linear) relaxation equation for magnetization. Two independent methods are used: direct measurement of the dynamic susceptibility of the mutual inductance bridge in weak probing field and measurement of heat release in the magnetic fluid at elevated magnetic field amplitudes. In the latter case, a well-known formula was used that relates the heat release power to the imaginary part of the dynamic susceptibility, which directly follows from the linear relaxation equation. The comparison of the results obtained by different methods revealed a systematic discrepancy between two series of experiments, which is explained by the existence in the magnetic fluid of multiparticle clusters (aggregates) with uncompensated magnetic moments. At frequencies of the order of 10 4 Hz, the Brownian mechanism of relaxation of the magnetic moments is blocked because of the large dimensions of the aggregates ( w t B >> 1). In addition, for small amplitudes of the magnetic field, the Neel relaxation mechanism is also blocked because of the high energy of interparticle interactions inside the aggregate and the appearance of high potential barriers. The situation changes with increasing field amplitude, when the energy of interaction of the aggregated particles with the external field becomes comparable with the height of the potential barrier. Aggregates are included in the process of magnetization reversal, increasing the dynamic susceptibility and the dissipation of energy in the magnetic fluid.