An amount of Ca2+ and PO43- ions transported into the enamel matrix change during tooth enamel crystals formation. Effects of the Ca2+ and PO43- ion flow on the lengthwise growth of octacalcium phosphate were studied at 37degreesC and at pH 6.5 in a model system of tooth enamel formation, where a cation selective membrane and a dialysis membrane were used to control the ionic diffusion. An amount of Ca2+ and PO43-ions that flow into the reaction space between the membranes was changed by using 5, 10, 30 mM Ca and PO4 solutions as ionic sources. The length increased while the width decreased with an increase in concentration of Ca and/or PO4 solutions. As a result, the length to width (L/W) ratio increased and the width to thickness (W/T) ratio decreased with an increase in concentration, resulting in narrower and longer ribbon-like crystals. It was suggested that growth of enamel crystals is, in part, affected by the change in the amount of Ca2+ and PO43- ions, that flow into the enamel matrix. (C) 2002 Elsevier Science B.V. All rights reserved.