Morphological aspects of dentinal caries in rats are described. Carious lesions on free dentinal lingual surfaces of lower first and second molars developed after: (i) removing the enamel layer by offering rats 0.1 M lactic acid as drinking water from day 20 to day 30; (ii) infecting the animals in relative gnotobiosis with Actinomyces viscosus OMZ 105E, Streptococcus mutans OMZ 176E and Lactobacillus acidophilus OMZ 3E in a complete factorial arrangement; (iii) feeding the rats a sucrose-rich cariogenic diet to the end of the experiment on day 94. The tissues from 2 animals of each group comprising 12 animals each were fixed by perfusion. The dissected jaws from these animals were decalcified in EDTA and further processed for light and electron microscopy. Material from the remaining decapitated animals was used for scanning electron microscopy and for the preparation of nondecalcified sections. The lactic acid treatment produced distinct dentinal windows. On day 30, the exposed dentin was practically unchanged with respect to density and structure of collagen and matrix. Only animals infected with A. viscosus alone or in combination with S. mutans and/or L. acidophilus produced dentinal carious lesions during the following caries test period which were morphologically characterized. Because of the close similarity between coronal and root dentin with respect to composition and structure, this animal model seems to be a valid approach to study the various aspects of root caries.