A method for the collection of animal foot pad sweat under a vapor barrier was presented. The average electrolyte composition of sweat from the cat foot pad was (in meq/l) sodium 175, chloride 145, and potassium 23.6. Using the same method applied to the human palm, the average sweat chloride concentration was 39.3 meq/l. On the basis of the above and the comparative morphology of the sweat glands of the cat and human, the production of hypotonic sweat is concluded to be a function of the duct of the sweat gland or perhaps the spatial relationship of the duct to the secretory coil.