1. Fluoride intoxication leads to sudden cardiac death which has been assumed to result from the accompanying severe hypocalcaemia. The aim of this study has been to investigate the suggestion that fluorapatite formation rather than CaF2 precipitation is responsible for this low calcium. 2. Measurements of free Ca2+ and F- ion concentrations in HEPES buffered solutions containing F-, Ca2+, and phosphate ions at different concentrations in the absence and presence of hydroxyapatite showed that the presence of hydroxyapatite enhanced the decrease of Ca2+ and F- concentration. 3. The ratio of Ca2+:F- clearance was 5:1 which is consistent with formation of fluorapatite. These results support the hypothesis that hydroxyapatite acts as a nucleation catalyst for fluorapatite formation and this process is responsible for the hypocalcaemia induced by fluoride intoxication. 4. The proposed mechanism explains also the metabolic acidosis which is frequently seen in cases of fluoride intoxication.