Exposure to oxygen deprivation in vitro has been reported to cause drug resistance in CHO cells (Rice et al., 1986; PNAS 83, 5978) and enhancement of experimental metastatic (colonisation) ability of murine tumour cells (Young et al., 1988; PNAS 85, 9533). Both these studies also demonstrated the induction of a subpopulation of cells with excess DNA content. Since the micromilieu in tumours results in exposure of the tumour cells to conditions of acid pH and nutrient deprivation, as well as hypoxia, we have examined the effect of exposure to acidosis (pH 6.5) and glucose starvation on drug resistance, cellular DNA content and the experimental metastatic ability of KHT sarcoma and B16F1 melanoma cells. Cells were exposed to these conditions for 24 and 48 h and tested for resistance to methotrexate (MTX) or experimental metastatic ability either immediately following these exposures or after 24 or 48 h of recovery in normal growth medium. Both cell lines demonstrated an enhancement of colonisation potential, which was most marked when cells were injected after 48 h of exposure followed by a 24 or 48 h recovery period. Flow cytometric analysis demonstrated an increase in the fraction of KHT cells with excess DNA following both glucose starvation and acidosis we observed only a small increase in MTX resistance following acidic exposure of cells and no change following glucose starvation. Since both acidosis and glucose starvation are known to induce glucose regulated proteins (grp), a subset of the stress protein family, we studied the effect of treatment with another known inducer, 2-deoxyglucose. We found that this agent affected the metastatic efficiency of KHT cells in a manner similar to that observed following exposure to glucose starvation and acidosis. However, further studies are required to establish what role, if any, grp play in this effect. In conclusion this study shows that transient exposure of murine tumour cells to an acidic or glucose deprived environment can cause progression in terms of metastatic potential.