During its later stages of development, the chicken embryo is oriented in the egg so that it occludes its left eye with its body and the right eye is positioned so that it can receive light input. This lateralized light input has been shown to play a decisive role in determining the direction of brain lateralization for two behavioural functions, copulation and performance of a visual discrimination task known as the 'pebble floor test', since the direction of lateralization for these functions can be reversed by occluding the right eye of the embryo on day 19/20 of incubation and at the same time exposing the left eye to light. The sensitive period during which this role of lateralized light input influences the lateralization for copulation extends to day 1 posthatching if the eggs are incubated and hatched in darkness, but it is over by day 3. For the pebble floor test the sensitive period is already over by day 1 posthatching. By exposing eggs to light for various times on day 19 of incubation, it was possible to determine that between 2.5 and 6 h of lateralized light exposure is necessary to stabilise the normal direction of lateralization so that it can no longer be reversed by occlusion of the right eye. Thus, in the developing chicken embryo substantial neural reorganization must occur in response to a brief period of lateralized light input.