The responsiveness of 6-day-old chicks varied in nature and degree according to their ongoing behaviour. Behaviour sequences were interrupted by a novel change, the illumination of a bulb, in the home pen. The behaviour of chicks following the novel change was compared with that of others whose ongoing behaviour was the same but which were undisturbed. The difference between these experimentals and controls, which we define as the response, involved fixation of the bulb and reduced feeding, general pecking and preening behaviour. Some birds showed much crouching and immobility whereas others showed more escape behaviour, for example jumping and peeping. A greater proportion of escape components and a smaller proportion of immobility components was included in the response if the chicks had been active, when the bulb was illuminated, than if they had been inactive. Specific differences in response according to ongoing behaviour included more crouching and less jumping by chicks which had been preening than by those which had been walking. These results and the fact that certain activities persisted briefly after bulb illumination are discussed in terms of the physical difficulties of change from one activity to another, attentional mechanisms, causal factors and the behavioural final common path. We propose that there is a 'locking on' mechanism which minimises inefficient vacillation between behaviours. Responsiveness can be very different according to ongoing behaviour, so this fact should be taken into account in any attempt to assess the responses of animals.