A study of the projection of root canals of natural teeth on dental roentgenograms was carried out as a sequel to an earlier phantom study. The material consisted of extracted teeth and teethin situin jaw preparations. The same radiation source and focus-film distance were used throughout, but the exposure times and tube voltages were varied. The roentgenographic images of the root canals were analysed densitometrically and compared with measurements of the actual object dimensions. The root were sectioned at the level studied and their cross sections were traced and measured with the aid of a profile projector.There was good agreement between the true breadth of the canal and that projected on the film. The maximum contrast in the image of the canal and the densitometrically measured difference in substance, expressed in metal equivalents, were proportional to the depth of the canal in the direction of radiation. The breadth of the canal in the plane of the film was the same regardless of whether the tube voltage was 50, 60 or 90 kV. On the other hand, the tube voltage affected the photographic density differences between the canal and the dentine walls. Changing the voltage from 50 to 60 kV did not affect the results, but there was a highly significant difference between 90 and 50 or 60 kV. The clinical significance of tube voltage and exposure time is discussed.