The baby, believed to be between six months and a year old, was in serious condition Tuesday.
Los Angeles police Lt. Scott Harrelson says the children's mother kicked the door in after hearing her older child, believed to be about two years old, calling for help.
She immediately pulled the baby out of the toilet, and she was blue but slightly breathing.
Harrelson says officers were able to open up her airways and she started to cry.
The baby was transported by helicopter to Children's Hospital Los Angeles for further treatment.