Dimethyl sulfoxide (DMSO) is widely and routinely used as a vehicle in various investigations, especially within the pharmaceutical industry. It has been used for the evaluation of the effects of hydrophobic xenobiotics on cells, as well as for the cryopreservation of biological material. Isothermal microcalorimetry is a powerful tool for monitoring heat production, which is a function of general cellular metabolic activity. Employing this microcalorimetric technique, a low concentration of DMSO routinely used for the addition of hydrophobic substances to, e.g., cell cultures, was shown to decrease heat production (per unit DNA) by the rat hepatoma cell lines FAO, Morris 7800C1 and H4IIE by 32-38%. However, such low concentrations of DMSO did not influence the cell cycle or the degree of apoptosis in these cell populations. Caution is thus advisable when utilizing DMSO as a vehicle in cell culture experiments.