The effects of compound I.C.I. 33828, a substituted dithiocarbamoylhydrazine, on fertility in the male domestic fowl have been studied. The daily introduction of the compound directly into the crop resulted in a gradual reduction in semen volume and fertility and after 2 or 3 weeks, depending on the dose, fertility dropped to zero. Seven days after giving medium and high dosages of the compound, watery droppings, loss of appetite, loss of body weight and weakness of the legs were observed in the treated cocks. All the effects were reversible since on withdrawal of the compound full recovery and normal fertility were achieved.