Recent, increasing interest in inositol tris(1,4,5)phosphate (IP3) turnover and metabolism has led to a need for a fast and quantitative determination of this compound in various tissues. These requirements are fulfilled by separation in different steps (FPLC and Sephadex G-10) culminating in further separation and quantification by isotachophoresis. Isotachophoresis means a migration of ion species of the same sign in an electrical field with all ions moving with the same velocity. The migration takes place when an electrical field is applied to a system of electrolytes of specific design. Detection was carried out by monitoring conductivity changes. The method is highly sensitive and allows measurements of IP3 in the pmolar range. Linearity was demonstrated over a wide range, 50-4500 pmol. The total imprecision was low with a coefficient of variation of 3%. When determining in biological tissue the recovery was estimated to be close to 100%. The mean content of IP3 in 15 rat heart specimens was 44.3 pmol/mg dry weight corresponding to tissue samples in the order of 4 mg wet weight. By noradrenaline stimulation myocardial content of IP3 increased by 50%.