Rats reared in the dark to 50 days show morphological and biochemical changes in the visual pathway. Light exposure results in elevated incorporation into protein in visual cortex, lateral geniculate and retina. Much of the visual cortex elevation is in a rapidly labelling, rapidly transported neuronal particulate protein. There are concomitant changes in lysosomal and transmitter enzyme activity. In chicks exposed to an imprinting stimulus (a flashing light) there are elevations in RNA polymerase and RNA and protein incorporation in the anterior forebrain roof (a.f.r.) compared with controls. There are changes in adenyl cyclase, cAMP and AChE. Behavioural controls show that although there are general biochemical sequelae of light exposure, the elevation in RNA synthesis in the a.f.r. is not a result of mo tor, stress or sensory activity, but is correlated wih a measure of the learning of the stimulus characteristics. A model for neurochemical correlates of developmental plasticity, learning, and statedependent transients is discussed.