IN a previous paper1, a method was described of producing enamel hypoplasia in the rat's incisor tooth by stopping matrix formation by the tall ameloblasts. This method consisted in essentials of first causing healing of low-phosphorus rickets and then suddenly stopping healing by re-imposing the rachitic condition. This procedure stops calcification of bone2 and dentin, but does not interfere with matrix formation in these tissues. In the case of enamel, however, matrix formation by the tall ameloblasts is stopped permanently, and these cells become temporarily seriously disoriented. The matrix and ameloblasts affected have been called the ‘experimental’ matrix and ameloblasts.