Smedslund, J. The acquisition of conservation of substance and weight in children. III. Extinction of conservation of weight acquired ‘normally’ and by means of empirical controls on a balance.Scand. J. Psychol., 1961,2, 85–87.—11 children who had acquired conservation of weight by control on a balance, and 13 children who had acquired conservation prior to the study, were shown some instances of apparent non‐conservation. All the children in Group I easily gave up their concept of conservation and reverted to more primitive perceptual strategies. About half of the subjects in Group II resisted the attempt at extinction and interpreted the apparent non‐conservation as meaning that something had been added or taken away. The results are seen as consistent with the hypothesis that a concept of conservation acquired by means of external reinforcement does not have the functional properties of a normally acquired ‘logically neces