The diuretic response to distension of a large balloon in the left atrium and the increase in discharge of atrial receptors for a given increment in left atrial pressure have been found to be greater in dogs with a high blood volume than in dogs with a low blood volume. To determine whether atrial size may influence atrial receptor discharge, a group of cats and two groups of dogs with different body weights were examined under anaesthesia with alpha-chloralose. The relationship between atrial receptor discharge and left atrial pressure was determined in twenty-six fibres from seven cats, thirty-seven fibres from eight small dogs weighing less than 15 kg and twenty-five fibres from five large dogs weighing more than 25 kg. The slope of the relationship between the increase in atrial receptor activity and increments in left atrial pressure was shown to be significantly greater in the large dogs than in the small dogs, and the slopes in dogs were greater than in cats. The size of the atria was greater in larger dogs than small, and both those groups had atria larger than those of the cats. These findings indicate that the response of an increase in atrial receptor discharge to the same increment in left atrial pressure was greater in large animals than small and that this difference is possibly due to the different atrial size.