Water transport across plant cell membranes is difficult to measure. We present here a model assay, based on chlorophyll (Chl) a fluorometry, with which net water transport across the cell membrane of freshwater cyanobacterium Synechococcus sp. PCC7942 (S7942) can be followed kinetically with millisecond-time resolution. In cyanobacteria, the phycobilisome (PBS)-sensitized Chl a fluorescence increases when cells expand (e.g., in hypo-osmotic suspension) and decreases when cells contract (e.g., in hyper-osmotic suspension). The osmotically-induced Chl a fluorescence changes are proportional to the reciprocal of the suspension osmolality (DeltaF proportional, variant Osm-1; Papageorgiou GC and Alygizaki-Zorba A (1997) Biochim Biophys Acta 1335: 1-4). In our model assay, S7942 cells were loaded with NaCl (passively penetrating solute) and shrunk in hyper-osmotic glycine betaine (nonpenetrating solute). Upon injecting these cells into hypo-osmotic medium, the PBS-sensitized Chl a fluorescence rose to a maximum due to the osmotically-driven water uptake. The rise of Chl a fluorescence (water uptake) was partially inhibited by HgCl2, at micromolar concentrations. Arrhenius plots of the water uptake rates gave activation energies of EA=4.9 kcal mol-1, in the absence of HgCl2, and EA=11.9 kcal mol-1 in its presence. These results satisfy the usual criteria for facilitated water transport through protein water pores of plasma membranes (aquaporins), namely sensitivity to Hg2+ ions and low activation energy.