SummaryParenterally-administered 2,4-D produces temporary myotonia in mice, rats, rabbits and dogs following a single injection. Repeated injections have failed to elicit either a characteristic chronic syndrome or a striking histologic picture; nor have they altered the process of reproduction and development in the dose ranges tested. The slight retardation of the growth rate of young mice on large daily doses is probably a manifestation of reduced food intake. Neoplastic growth has not been inhibited.The author is grateful to Dr. Joseph C. Aub for the suggestion of this problem and for encouragement and advice. The technical help of Amory Glenn is also greatly appreciated.