If a fluorogenic compound, such as fluorescein diacetate, is added to a water solution containing living cells it becomes hydrolyzed by intracellular esterases into a fluorochrome whose fluorescence can be used to monitor the cytoplasmic pH and the cytoplasmic viscosity of the cells. In this paper we have used this technique to measure the effects of different concentrations of Co2+ and Cd2+ ions on the cytoplasmic pH and the cytoplasmic viscosity of a single cell culture. Our results indicate that the observed decrease in the efficiency of the intracellular hydrolyzation of fluorogenic substances in the presence of different concentrations of heavy metals could be caused by both a decrease in the cytoplasmic pH and an increase in the cytoplasmic viscosity. A decrease in cytoplasmic pH would decrease the effectiveness of the intracellular enzymes, whereas an increase in cytoplasmic viscosity would decrease diffusion which would also reduce the effectiveness of the reaction. The dependence of the reciprocal of the cytoplasmic viscosity on the concentration of these metals correlates well with published results on their toxicity.