Carbachol stimulation of muscarinic receptors in rat cortical slices prelabelled with myo-[2-3H]inositol caused the rapid formation of a novel inositol polyphosphate. Evidence derived from its chromatographic behaviour, and from the structure of the products formed in partial dephosphorylation experiments, suggests that it is probably D-myo-inositol 1,3,4,5-tetrakisphosphate. An enzyme in human red cell membranes specifically removes the 5-phosphate from it to form inositol 1,3,4-trisphosphate. It is suggested that inositol 1,3,4,5-tetrakisphosphate is likely to be a second messenger, and that it is the precursor of inositol 1,3,4-trisphosphate and possibly of inositol 1,4,5-trisphosphate.