The fluorescence lifetime of fluorescent proteins is affected by the concentration of solutes in a medium, in inverse correlation with local refractive index. In this paper, we introduce the concept of using this dependence to probe cellular molecular environment and its transformation during cellular processes. We employ the fluorescence lifetime of Green Fluorescent Protein and tdTomato Fluorescent Protein expressed in cultured cells and probe the changes in the local molecular environment during the cell cycle progression. We report that the longest fluorescence lifetimes occurred during mitosis. Following the cell division, the fluorescence lifetimes of these proteins were rapidly shortened. Furthermore the fluorescence lifetime of tdTomato in the nucleoplasm gradually increased throughout the span of S-phase and remained constantly long until the end of interphase. We interpret the observed fluorescence lifetime changes to be derived from changes in concentration of macromolecular solutes in the cell interior throughout cell cycle progression.