Testosterone-treated chicks (T's) were more persistently disturbed than controls (C's), both by the introduction of a novel object into the home cage, and by its removal 24 hours later. This was shown, firstly, by delay in the first appearance of behaviour not directed to the novel object: the preliminaries to feeding (looking at the floor, visiting the food hopper) appeared later in T's, as did drowsiness. Secondly, behaviour was interrupted in T's by a return to investigation: in particular the first bout of feeding was markedly abbreviated and (after introduction of the object) was not followed as usual by stretching. None of these differences between T's and C's were present in undisturbed behaviour. It is probable that the development in T's of unusually protracted periods of sleep after removal of the novel object is another consequence of more prolonged disturbance in T's. Other evidence is cited suggesting that protracted sleep may be caused in this way. Increased persistence of attention, including an ability to return to a previous point of attention after a shift to a distracting stimulus, is known from other studies to be produced by testosterone in male chicks. Such an effect would explain the above findings. It is argued more speculatively that it may also underlie differences between T's and C's in the character of investigation, and not just its prolongation.