Inositol polyphosphates other than Ins(1,4,5)P3 are involved in several aspects of cell regulation. For example, recent evidence has implicated InsP6, Ins(1,3,4,5,6)P5 and their close metabolic relatives, which are amongst the more abundant intracellular inositol polyphosphates, in chromatin organization, DNA maintenance, gene transcription, nuclear mRNA transport, membrane trafficking and control of cell proliferation. However, little is known of how the intracellular concentrations of inositol polyphosphates change through the cell cycle. Here we show that the concentrations of several inositol polyphosphates fluctuate in synchrony with the cell cycle in proliferating WRK-1 cells. InsP6, Ins(1,3,4,5,6)P5 and their metabolic relatives behave similarly: concentrations are high during G1-phase, fall to much lower levels during S-phase and rise again late in the cycle. The Ins(1,2,3)P3 concentration shows especially large fluctuations, and PP-InsP5 fluctuations are also very marked. Remarkably, Ins(1,2,3)P3 turns over fastest during S-phase, when its concentration is lowest. These results establish that several fairly abundant intracellular inositol polyphosphates, for which important biological roles are emerging, display dynamic behaviour that is synchronized with cell-cycle progression.