The internal elastic lamina (IEL) of normal canine carotid arteries was examined by scanning electron microscopy in pressure-fixed specimens with intact endothelium. IEL appearance showed a marked variation between animals and was classified into fenestrated sheet, fibrous, and mixed varieties. This interpretation of the apparent morphology was confirmed with transmission electron microscopy. It was clear that IEL fenestrae were associated with surface depressions and that in areas of fibrous IEL there was surface elevation over individual fibres. Within individual animals there was little variation in the pattern of IEL either along or between common carotid arteries. If theories of atherogenesis involving the IEL are correct, the variation of IEL patterns between animals would suggest a corresponding variation of incidence and severity of atheromatous lesions of the common carotid artery between animals. Further, the occurrence of fibrous areas distributed throughout the fenestrated sheet would suggest a focal distribution of lesions along such arteries.