Sixty-gramme rats were given either 0, 75, 100 or 150 parts/10(6) fluoride in their drinking water. After five weeks, the fluoride, the phosphorus and the protein contents of the enamel were compared in control and experimental animals at three stages of enamel development. The mineral content was reduced in pigmented enamel from animals given 75 parts/10(6) or more fluoride in their drinking water. The fluoride content was elevated in all stages of fluorosed enamel development. At the lowest fluoride level (75 parts/10(6], a larger proline content was found in the proteins of the maturing, fluorosed enamel but there was no increase in the protein content. In animals given 100 parts/10(6) fluoride in their drinking water, the proline content of the protein was greater in maturing, fluorosed enamel, and the total protein content of the post-secretory enamel (maturing and pigmented) was greater than in the controls. These observations indicate that, with increasing levels of fluoride in drinking water, there was an initial delay in the loss of the amelogenin proteins followed by a decreased removal of total protein from the enamel. These results indicate that fluoride interfered with the normal post-secretory, pre-eruptive development of enamel.