The concentrations of the two brain specific proteins, 14-3-2 which is predominantly neuronal, and S-100 which is glial, have been determined in the optic tectum of a series of chick embryos between the 3rd day of incubation and the 9th week post-hatching. Both proteins first appear during the 4th day of incubation at about the time the first differentiated cells can be recognized morphologically. The level of the two proteins shows little increase during the ensuing 8 days at which time cell proliferation ceases, but between about the 17th day of incubation and the 2nd week after hatching the concentration of both proteins increases more-or-less exponentially. Thereafter the rate of increase declines rapidly, but there is a slow continuing increase for some weeks until adult levels are reached. Although the changes in concentration of the two proteins show a close parallelism, the concentration of 14-3-2 is approximately 100 times greater than that of the S-100 protein at each stage in development.