The breadth of responsiveness of rat taste cells to the four basic taste stimuli was studied using the entropy measure (H) proposed by Smith and Travers. H values range from 0.0 for narrow tuning to 1.0 for broad tuning. Based on the responses of depolarizing receptor potentials of 26 rat taste cells to the four basic taste stimuli, taste cells were classified into nine NaCl-best, four Q-HCl (quinine-HCl)-best, 10 HCl-best and three sucrose-best cells. NaCl-best cells were narrowly tuned to the four basic taste stimuli, but the other three stimuli-best cells were broadly tuned to the stimuli. In all, 85% of the taste cells responded to more than one of four basic taste stimuli. The mean H values for NaCl-best, Q-HCl-best, HCl-best and sucrose-best cells were 0.285, 0.832, 0.781 and 0.796 respectively. The mean H value for all 26 taste cells was 0.621. This was larger than H in rat gustatory fibers. Transformation of large H values in taste cells into small H values in taste fibers may be due to a non-random interaction between taste cells and taste fibers during the synaptic formation. Broad tuning properties of rat taste cells suggest that the across-taste cell response pattern may play an important role in taste quality coding mechanisms.