Abstract: Methods and pharmaceutical compositions are disclosed for reversibly inhibiting sperm receptor activity in animals. Nicarbazin, its derivatives and modifications which retain pharmacological activity are shown to inhibit activity of zona pellucida proteins and concomitant synthesis and/or assembly of the sperm receptor on the oocyte surface necessary for fertilization. Nicarbazin is easily administered, for example by simple addition to feed of an animal and is and non-toxic to the animals, providing a safe and efficient means for controlling populations of mammals and avian species.