A single, high dose of 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) results in three different types of lesions along the enamel surface of the rat incisor, one of which is seen as a "bright band" crossing the final enamel surface in the scanning electron microscope (SEM). The present study presents the structural surface features of final enamel formation and its subsequent maturation in normal and HEBP-exposed rats. The position of the bright band is examined in relation to where the Tomes processes pits disappear (DTPP), where the boundary between "light" and "dark" enamel (LDB) as seen by SEM is located, and in particular, where the so-called opaque boundary (OB) is positioned. Groups of rats were given a subcutaneous dose of 0, 5, or 10 mg P/kg body wt of HEBP and killed at intervals of either 12 hours or 2 or 9 days. The mandibular incisors were processed for SEM after enzymatic digestion of enamel organ remains. Enamel surface nodules, 100-300 nm in diameter and composed of smaller units, were evident at the start of final enamel formation which was defined as the area from DTPP to LDB. With increasing maturation, the nodules merged to form a smooth surface. In HEBP-treated animals, growth and merging of these surface nodules became arrested at the time of injection resulting in an irreversible "porous" stage corresponding to that part of the surface enamel. This area--the bright band--developed corresponding to the start of the area of final enamel formation and was subsequently carried incisally during the eruption of the incisor.(ABSTRACT TRUNCATED AT 250 WORDS)