In a recent manuscript, appearing in the present issue of this joumal[l], Rivlin engaged in a lengthy commentary on the endochronic theory. The reader who is familiar with the theory knows that the latter owes its inception to a sequence of two papers by Valanis, published in the Archives of Mechanics in 1971[2,3]. The physical basis of the endochronic theory lay in the theory of irreversible thermodynamics of internal variables and its purpose was the development of explicit constitutive equations for modeiling plastic or viscoplastic behavior of materials subjected to small or large deformation. The motivation for developing such a theory was two-fold and consisted first of a desire on my part to develop an alternative theory of plasticity which did not require the assumption of yield for its development, and second of an undertaking to bring the constitutive theory of plastic behavior under the aegis of irreversible thermodynamics of internal variables, a task which theretofore had not yet been accomplished. Since its inception, the theory has undergone sign&ant evolution, the driving force always being the desire to describe as many aspects of material behavior as possible with the simplest possible and most elegant form of a constitutive equation. It is therefore quite odd, to say the least, that Rivlin’s remarks are, in the main, addressed to the earliest versions of theory. His comments on the later developments are scant, inaccurate and amount to no less than a total misrepresentation of the work. Let it be said, however, that his comments on the earlier work are hardly better. Rivlin’s barrage of criticism can be divided, for the purposes of rebuttal, into fiw categories. (i) Errors of fact. (ii) Contentions and unsubstantiated criticisms and general charges without foundation. (iii) Presumed shortcomings of the theory vis-a-vis criteria which are either generally invalid or unsubstantiated. (iv) Perceived peculiarities of the theory in terms of mathematical constraints which have no physical foundation. (v) Differences in style and general philosophy, regarding the purposes and scope of the field of constitutive equations. For the purposes of enlightenment of the reader and to set the grounds for the subsequent discussion I would like to point out at the outset that there is no such thing as a unique endochronic theory of constitutive behavior. The term “endochronic theory” encompasses all those theories in which “fhe state of stress at the present time is a function of the history of strain with respect to a lime scale, which is not the absolute time scale measured by Q clock, but a time scale which in itself is a ptvpetty of the material at hand” [2]. For instance the classical linear theory of viscoelastic materials is not an endochronic theory because the “material memory” therein is defined with respect to the absolute time scale measured by a common clock. The “arc length” theory originally proposed by Ilyushin[4] and elaborated upon by Rivlin[5] and Pipkin[S, 131 is not an endochronic theory since the material memory scale “s” introduced