Steady-state and time-resolved emission studies on Nile Red, a biological fluorescent probe, in homogeneous solutions and in zeolite 13X and LZY are reported. It is shown that at moderately high polarity the main nonradiative pathway of Nile Red is twisted intramolecular charge transfer (TICT) process, the rate of which increases exponentially with increase in polarity (E(T)(30)) of the medium. Due to this polarity-dependent TICT process Nile Red is a sensitive indicator of the micropolarity of biological environments. From the comparison of the emission properties of Nile Red in zeolite X and Y with those in homogeneous solutions the E(T)(30) values of zeolite 13X and LZY are estimated to be 55.5 +/- 1. This indicates the polarity of the zeolites is similar to that of an aqueous methanol solution.