Mayor Antonio Villaraigosa signed the new law last year in an effort to save about 100,000 dogs and cats from being put down over a five-year span.
Pet breeders opposed it statewide, but the law was still passed in the city of Los Angeles.
Authorities said animal services will not be going door-to-door, and they will not be scoping out animals at parks, but they do intend to enforce it by checking out animals as they go on animal complaint calls.
Pet owners caught with an unsterilized pet will get 60 days to do it, and if it isn't done in that time, owners will face a $100 fine or community service.
A third offense could result in a $500 fine, more community service and a possibility of a misdemeanor charge.
There are exceptions for animals who compete in shows or sporting events.
The Spay L.A. Initiative hopes to fix 500,000 dogs and cats over five years.
L.A. City Controller Laura Chick said in an audit that L.A. Animal Services was ill-prepared to handle this, but animal service said it is hoping that word of mouth will help.
For pet owners worried about paying for spaying or neutering, Spay Day L.A. features a free sterilization clinic from October 24 through the 26 at animal shelters throughout the city.