Purina Cat Chow: How Often Do Cats Come into Heat?
- Explains how often cats come into heat and how long the heat period is.
- http://catchow.com/catcarecenter_3_adult_1.aspx?combinedID=20400

Animal Planet: Adolescence and Sexual Maturity
- Describes the stages of development in a cat's life from kittenhood to full sexual maturity.
- http://animal.discovery.com/guides/cats/body/maturity.html