Thermoelastic measurements at constant volume are reported for a series of natural rubber samples. The energy component of the stress supported by the network is more or less independent of the network cure at a value of fe/f= 0.12±0.02. The energy component of the stress is independent of whether the measurements are made in the dry or in the swollen state, despite the fact that the dry rubbers have non-Gaussian equations of state and that the swollen rubbers approach Gaussian behaviour. Flory's analysis of rubber elasticity which includes hindered internal rotation in the main polymer chain, is compared with experimental results. To a first approximation it gives the correct order of magnitude for the energy component of stress. Measurements have also been made of the pressure coefficient of stress, from which the dilation coefficient of the rubber has been calculated as a function of strain. Flory's analysis does not appear to predict this coefficient satisfactorily.