IN some species, restriction of sensory experience during early postnatal life can affect functional connectivity within the brain1. Yet, in these circumstances, little is known of the changes that occur to the internal organisation of brain cells. It is also significant that certain membrane components of brain cells are continuously turning over and some of these are responsible for imparting to the membrane and cells their specialised functions2. Thus plastic changes may sometimes result from a local alteration in the number or nature of molecules available for incorporation into the membrane, and it then becomes relevant to consider those systems which regulate the differential distribution of materials needed for maintenance and growth of cellular processes and synaptic junctions. Consequently, both here and in previous papers3,4 we consider the possibility that the turnover of microtubular protein (tubulin) may be involved in some of the plastic changes in the brain that result from learning or early experience. We report here significant changes in tubulin synthesis that are correlated with eye opening and the duration of a critical period.