The paper describes researches which have been carried out at the National Physical Laboratory for the Radio Research Board. An improved method of measuring the amplification of a radio receiver is described, together with the manner in which this method can be applied to practical cases, particularly at short wavelengths. The apparatus covers a wavelength range of from 15 to 30 000 metres, and is capable of dealing with inputs of from 0.25 to 1 000 mV over this range. After describing in detail the construction and operation of the apparatus in Section (1), the results of tests on four receivers of widely different types are given in Section (2), and some general deductions are obtained from the properties of these receivers. In conclusion, suggestions are made in Section (3) for a possible means of classifying receivers for different purposes.