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1. The rate of fluid movement across the wall of isolated amphibian urinary bladder was estimated using hemi-bladders of the tropical euryhaline Rana cancrivora, the temperate R. temporaria and the tropical Bufo melanostictus.

2. The rate of water outflow across the bladder walls was: Rana cancrivora, 4·8 µl./cm².hr; R. temporaria, 7·2 µl./cm².hr; Bufo melanostictus, 3·7 µl./cm².hr.

3. When neurohypophysial peptides were added to the serosal solution the rates of water outflow were increased; dose—response curves were prepared relating the augmented rate of fluid movement to the concentration in the bathing fluid of vasopressin, oxytocin or vasotocin.

4. The potencies of the peptides in respect of bladders of each species were:

Vasopressin, B. melanostictus:Rana temporaria:R. cancrivora, 500:20:1.

Oxytocin, R. temporaria:B. melanostictus:Rana cancrivora, 100:5:1.

5. The ratio of sensitivity of the bladder of each species to the presence of the different peptides was:

R. cancrivora, vasotocin:oxytocin:vasopressin, 1000:5:1.

R. temporaria, oxytocin:vasopressin, 15:1.

Bufo melanostictus, oxytocin:vasopressin, 1:30.

6. The effect of extract of the pituitary gland of Rana cancrivora was examined on R. cancrivora bladder. It had a dose—response curve similar to the dose—response curve of vasotocin.

7. The vasotocin content of the pituitary gland of R. cancrivora was estimated

(a) from the dose—response curves using R. cancrivora bladder,

(b) by a three-point assay using R. cancrivora bladder and

(c) by its effect on water uptake across the skin of Bufo melanostictus. One pituitary gland of Rana cancrivora was found to contain about 0·1 µg vasotocin.

8. The urinary bladder of R. cancrivora was less responsive to neurohypophysial peptides than the bladder of R. temporaria or Bufo melanostictus.

9. The hormonal content of the pituitary gland of Rana cancrivora was smaller than that found in other amphibians.