Thy-1 is a developmentally regulated cell surface glycoprotein, found at high levels in nerve and certain other tissues. In brain its level rises nearly 100-fold during early postnatal development, but until recently the cellular basis of this, or even the distribution of Thy-1 in adult nervous tissue, has been unclear. On the other hand considerable progress has been made in elucidating the gene, and protein and carbohydrate structure of this molecule, so that the basis for its developmental regulation, and possibly its function, is beginning to emerge. In adult nervous tissues Thy-1 appears to be a surface component of most neurons, although within the CNS the density of antigen varies considerably on different cell types. Early in development, however, Thy-1 is present on only certain neurons--for instance on Purkinje cells in cerebellar cortex--and Thy-1 immunohistochemistry can demonstrate new aspects of the development of such cells. During development, previously Thy-1-negative cells can acquire high levels of Thy-1, and conversely detectable antigen can be lost from cells which at earlier stages stained intensely in Thy-1 immunohistochemistry. Such developmental changes can occur in waves of maturation which are defined spatially as well as temporally. Changing patterns of Thy-1 expression are also found at some sites in nervous tissue after axonal injury. The evidence for a role for Thy-1 in nervous system development is discussed, both in terms of its changing distribution during development and in the light of more direct attempts to define its role on the cell surface.