Androgens appear to cause reduced distractibility and increased focussing of attention in a number of species of mammals and birds. It is here shown, using runway tests, that in the domestic chick the changes in attention produced by testosterone are diverse, affecting on the one hand processes following immediately after perception, and on the other the allocation of attention between different stimuli over quite long periods of time. Examples of the first type include reduced distraction at first presentations of a conspicuous but irrelevant stimulus, increased response to change in the stimulus on which attention is focussed, and increased likelihood of fast response when a stimulus of the type which is being sought, appears in an unexpected position. All can be explained as due to an increased ability to specify stably a particular stimulus or type of stimulus as that which is being sought or on which attention is to be sustained. Longer term changes in the organisation of behaviour suggest that more than one focus of attention can be stably specified in a particular period of time. Testosterone-treated chicks will alternate attention between two very different stimuli (food and a conspicuous novel stimulus), even when one cannot be seen from the other without locomotion. It is necessary to consider the full complex of linked changes which may result from modulation of attention by hormones (or other physiological variables), in any discussion of the benefits and costs which might result.