The breadth of the responsiveness of gustatory neurons to the four basic taste stimuli (sucrose,NaCl, HCl and quinine hydrochloride) is a question of importance to current theories of gustatory quality coding. The mathematical expression of entropy from information theory can provide a measure of the breadth of tuning of these neurons. The entropy measure, as applied here, varies continuously from 0.0 for a unit that responds exclusively to one stimulus (i.e., narrowly tuned) to 1.0 for a cell that responds equivalently to all four of the basic compounds (i.e., broadly tuned). Subtle variations in the neural response profile of a cell, such as those produced by changes in stimulus concentration, are reflected in this measure. Thus, the use of the equation for entropy to describe the responsiveness of gustatory neurons provides a quantitative measure of their breadth of tuning that can be meaningfully applied to the problems of gustatory quality coding.