The purpose of this study was to evaluate histologically the extent and characteristics of the penetration of a periodontal probe during clinical assessment of loss of periodontal attachment in three different situations: (1) experimental mild gingivitis, (2) experimental severe periodontal inflammation and (3) experimental moderate periodontal inflammation. Mild gingivitis was obtained by merely allowing plaque to accumulate on the teeth. Severe periodontal inflammation was created using copper bands for a period of 3 weeks followed by placement of cotton ligatures for another 11 weeks. Moderate periodontal inflammation group was obtained by allowing the experimental defects to recover for a period of 3 to 31 weeks. At different times gutta percha imitations of a thin periodontal probe were inserted into the pockets using a gentle but unknown force. Histologic observation in 30 specimens showed that epithelium was always present around the probe tip, in most instances forming a continuous layer of epithelial cells. Histometrical analysis showed that in mild gingivitis the probe tip failed to reach the apical termination of the junctional epithelium (mean = -0.84 mm). In severe periodontal inflammation the tip of the probe went past this point (mean = +0.50 mm), while in moderate periodontal inflammation the probe tip came closest to the apical termination of the junctional epithelium (mean = -0.05 mm). It is concluded that the epithelial lining of a pocket stays intact, even in severe periodontal inflammation where the probe tip is situated apical to the apical termination of the junctional epithelium, indicating that during clinical probing the periodontal tissues are compressed and displaced but not perforated. It is also concluded that in beagle dogs the extent of probe penetration in experimentally inflamed periodontal tissues is dependent upon the degree of inflammation.