Spaying (ovario-hysterectomy) is the surgical removal of the reproductive organs (ovaries, uterus, fallopian tubes) of the female animal. Neutering (orchectomy or castration) is the surgical removal of the reproductive glands (testes) of the male animal. The outer is left, only the testes are removed. Appearance depends upon the dog's age at the time of the surgery. Females and males should be spayed or neutered by 6 months of age. Currently some clinics are performing surgeries on animals as young as 8 weeks of age. As this procedure becomes more common, perhaps it will be available in all areas. Older animals can be done as long as they are in good health. All sterilization surgery is performed under general anesthesia by a licensed veterinarian.

Female dogs and cats can be spayed when in heat or pregnant. This can usually be done up until a few days before delivery. These surgeries can take longer, and can therefore cost more. Spaying before having a first litter or heat cycle is usually a simpler procedure. The heat cycle for dogs is once or twice a year starting as early as 6 months of age. Duration is 3 weeks. Heat cycles in cats start as early as 6 months and occur every 3-4 weeks during spring through early fall. The gestation period for both dogs and cats is 63 days. Female cats can become pregnant again as soon as 10 days after giving birth (while still nursing the first litter).