Phytate (inositol hexaphosphate; InsP6) was determined in rat tissues fed on diets with different phytate contents, using a GC-mass detection methodology that permitted the evaluation of the total amount of this substance present in such tissues. The highest InsP6 concentrations were found in brain 5.89 x 10(-2)(SE 5.7 x 10(-3)) mg/g DM), whereas the concentrations detected in kidneys, liver and bone were similar to each other 1.96 x 10(-3) (SE 0.20 x 10(-3), 3.11 x 10(-3) (SE 0.24 x 10(-3), 1.77 x 10(-3) (SE 0.17 x 10(-3)) mg/g DM respectively) and 10-fold less than those detected in brain. When rats were fed on a purified diet in which InsP6 was undetectable, the InsP6 levels of the organs mentioned earlier decreased dramatically (9.0 x 10(-4), 3.8 x 10(-5), 1.4 x 10(-5) mg/g DM in brain, kidneys and liver respectively) and in some cases became undetectable (bone). The addition of InsP6 to this purified diet led to the increase of InsP6 levels in these tissues. This clearly demonstrated that the majority of the InsP6 found in organs and tissues has a dietary origin and is not a consequence of endogenous synthesis. Consequently, considering that InsP6 could be involved in some important biological roles, the value of any diet on supplying this substance is noteworthy.