The application of automatic frequency control to the Robinson magnetic resonance absorption circuit is described. The main advantage is the almost complete elimination of the effects of spurious amplitude modulation when frequency modulation and sweep are employed. Also the oscillation level remains constant during sweep, and large sweep amplitude may be employed even at low oscillation levels. The relative merits of phase-sensitive detection at the first and second harmonics of the modulation frequency are discussed with particular reference to the study of the nuclear magnetic resonance of ⁵⁷Fe in zero external field.