Injection of mammalian anti-diuretic hormone (ADH) into toads produced increased water uptake and decreased urine production, resulting in weight gain over the pre-injection period. A bioassay for ADH, based on this response, has been developed for Bufo marinus, a large toad available in the United States. This method is an adaptation and modification of an assay devised by Buchborn for the European toad Bufo bufo. Details of the assay procedure are given, as well as instructions for maintaining the toads in the laboratory.