1. The effects of extracts of the posterior lobe of the pituitary on the body water of the developmental stages of the toad, Bufo bufo bufo (L.) have been investigated by injecting extracts and weighing the animals at short intervals afterwards.
2. No response was shown until marked absorption of the tail had occurred. From this stage onwards it became progressively established and gradually increased in magnitude. The adult Salientian type of response was only elicited at the very end of metamorphosis.
3. By staining tadpoles with phenol red and measuring the rate of elimination of this dye, it has been shown that the action of the extracts is not antidiuretic except in completely metamorphosed animals.
4. It is provisionally concluded that the extracts have a double effect, first to increase the rate of water uptake and second, to diminish water loss. During the life-history of the toad, the first response is established when tail absorption is progressing rapidly, i.e. at a comparatively early stage, whereas the latter only comes into play at the very end of metamorphosis.
5. Data at present available suggest that an influence of the posterior pituitary on body water is characteristic of terrestrial but not of primarily aquatic vertebrates.