Evidence in the past year has provided support for a prominent role of tyrosine phosphorylation in the regulation of neuronal function. The discovery that many novel forms of protein tyrosine kinases and phosphatases are expressed in the brain has revealed that the regulation of tyrosine phosphorylation is highly complex. The recent identification of substrate proteins in the brain for the protein tyrosine kinases and phosphatases has begun to clarify the functional role of tyrosine phosphorylation in the development and modulation of the nervous system.