Thhe linearized theory of Gruler et al. , which gives the initial slopes of the capacitance voltage (C. V. ) and capacitance magnetic field intensity (C. H. ) curves of homogeneous, parallel aligned nematic samples with zero surface tilt, has formed the basis of many determinations of their splay and bend elastic constants. This letter describes an investigation of the range over which this theory is valid and discusses the implications of findings for the experimental determination of the elastic constants of nematic materials.