Disclosed is an improved method for measuring the volume of an animal paw, suitable for use in a rat paw edema assay. Also disclosed is a novel apparatus for measuring the volume of liquid displaced by the insertion of an object into a liquid filled cup (1) which is on a cup holder basin (3), the liquid filled cup and cup holder basin being inside a rectangular support assembly (5), the measurement obtained by determining the buoyant force produced as a result of the displacement of the liquid by the animal paw.