Three types of macroscopic incisor abnormality were observed in Wistar rats examined post mortem. Most common was a n altered relationship of the incisors to one another due to either fracture, elongation, rotation, malposition, or a combination of these factors affecting one or more incisors. Enamel hypoplasia was evident as opaque whitish blotches, bands or fine transverse striations. Other incisors showed an overall uniform pallor of the enamel. These abnormalities frequently affected the same animal or the same incisor. The associations between them were significant (P < 0.01). All the incisors of the intact group were normal and the incidence of abnormalities in hypophysectomized rats was not significantly higher (P > 0.05). However, of the thyroparathyroidectomized animals 56.8% had an abnormal incisor relationship, 28.4% showed discrete enaqel defects and 23% showed enamel pallor. The incidence of all types of abnormality was higher in surgically thyroparathyroidectomized rats (P < 0.05) than in other groups. No significant differences were found between completely and incompletely thyroidectomized rats or between subgroups which received different drug treatments. These findings are discussed in relation to similar reports and hypotheses are advanced to explain the abnormalities. Parathyroid deficiency rather than thyroid deficiency appears the most important etiological factor.