The responses of chicks startled near the end of a bout of feeding or preening were different from, and measurably greater than, those of chicks startled at the beginning of bouts of these activities. This result provides evidence about changes in motivational state during a bout of activity. It is proposed that causal factors, which are changing during the bout, may initiate an attentional change as the point of transition to another activity approaches. Distractibility and information-processing rate may be increased near the end of an activity because the individual can then attend to a greater variety of inputs than it could earlier in the bout. This would maximise the efficiency of the ensuing behavioural change. In order to carry out these experiments, bouts of feeding and preening were measured using a criterion based on log survivor curves for inter-event gap-lengths. The effects on the response of the time during a bout at which interruption occurs, were shown by chicks of 2, 4 and 6 days of age. The method used in these experiments, the interruption of activities at previously defined instants whilst observing behaviour in detail, affords opportunities for studying the nature of behavioural sequences and their control.