Experiments described which were designed to test the hypothesis that the ease with which rats will shift attention between stimuli varies with the oestrous cycle. It was predicted from the results of earlier work that at oestrus, animals will shift attention less readily than at dioestrus, provided that attention is already focussed on a specific stimulus. Data from two-stimulus tests, and runway tests confirmed this prediction. It is suggested that the nature of the underlying changes may be similar to those induced in young chicks by exogenous testosterone, and may involve changes in hippocampal function.