One hundred and thirty-eight post-graduate students enrolled in a Diploma in Education course were tested for their ability to converse weight and volume. A methodological criticism of the earlier work or Elkind (1962) and Towler & Wheatley (1971) is offered which casts doubt on the extremely high proportion of non-conservers of volume reported in those studies. Nevertheless, using a modified version of Elkind's methodology, 18 per cent of our sample were non-conservers of volume, a result providing some support for the generaltiy of Elkind's findings. The results are discussed under the four headings of horizontal decalage, sex differences, possible effects of pre-graduate courses of study and educational implications. The importance of the individual as constructor and interpreter of his own experience is emphasized.