It is shown that an averaged learning equation in neurodynamics is an integrable gradient system having a Lax pair representation.In this decade, applied analysis based upon the theory of nonlinear integrable systems has been gradually developed. Though the theory has its origin mostly in classical mechanics, this methodology supplies a new mathematical tool which can be widely applied to:i) linear prediction problems [13], (ii) the geometry of linear systems [10, 14], (iii) maximum likelihood estimation [4, 15], (iv) matrix eigenvalue problems [7, 16, 24], (v) the information geometry [17, 18], (vi) linear programming problems [5, 19],and so on. Here (i), (ii) and (vi) are essentially nonlinear problems being due to certain quotient structures and boundary conditions. The motivation of applied analysis of integrable systems is explained in a review article [20]. Although it looks accidental, the dynamical theoretic approaches to the variety of interesting problems turn out to have properties in common. Namely, the dynamical systems which have emerged are integrable systems of Lax type. Rich information about integrable systems successfully helps us to solve the problems being considered as well as the dynamical systems themselves. Solvability of the original nonlinear problems may be related to integrability of the resulting dynamical systems at a deep level.In the following sections, we consider (vii) neurodynamics as a new topic of applied analysis of nonlinear integrable systems. We here take the position that we regard the problem of learning as a challenging problem in nonlinear dynamical systems. First we discuss a stochastic learning equation of Hebb type with a nonlinear *