This invention relates to the novel identification of arginase as an enzymatic activity which can reverse inhibition of neuronal regeneration in the central and peripheral nervous system. Assays to monitor the effects of various agents on arginase expression and thus on neuronal regeneration and repair and to identify agents which will block or promote the inhibitory effects on neuronal outgrowth are provided. This invention also relates to compositions and methods using agents that can reverse the inhibitory effects of myelin on neural regeneration by affecting arginase activity or putrescine and derivative polyamine levels in a neuron. Methods for regulating and for promoting (or repressing) neuronal growth or regeneration in the nervous system, methods for treating injuries or damage to nervous tissue or neurons, and methods for treating neural degeneration associated with conditions, disorders or diseases, comprising the step of administering at least one of the compositions according to this invention, are provided.