Experiments have been conducted on maternal-nest building and factors affecting its occurrence in the rabbit. Females deprived of nesting material built a nest out of their own body hair and were as successful in rearing young as does which used both body hair and straw; females deprived of body hair (by shaving) and nesting material showed defective maternal behaviour. Interrupting pregnancy by spaying or foetectomy resulted in maternal-nest building. Nest building was induced in spayed females by the withdrawal of progesterone after injections of progesterone and oestradiol for several weeks. Similar regimes did not induce nest building by castrated males. Further experiments strongly suggest that the onset of nest building is governed by the change in the ratio of progesterone to oestrogen.