Much attention has been devoted in the past to monitor changes of mobile lipid (ML) (1)H MRS signals in spectra of tumour cells. The purpose of this work is to exploit ML signals to provide information on cell metabolism after irradiation, comparing tumour cells characterised by different radiosensitivity and relating MRS findings to changes in cell proliferation and delays in cell cycle phases. Irradiated HeLa cells present less intense ML signals with respect to controls. The opposite is true for MCF-7 cells. A G(2) arrest is observed for both cell lines after irradiation. In HeLa cells, G(1) decreases and S phase is maintained; a sub G(1) peak is also visible. In MCF-7 cells, G(1) is decreased and S phase is strongly reduced, while no sub G(1) is present. The observed changes in ML are tentatively associated to cell cycle regulation of phospholipid synthesis. Mathematical modelling of ML variations is in progress.