What Can I Do If My Cat Ate Transmission Fluid?

This cat needs to be at a veterinary clinic as soon as possible. Emergency clinics are open 24 hours and on holidays. Your veterinary office will likely list the closest emergency clinic on their answering machine or web site. Some veterinarians have "on-call" duties and will come in to see emergency cases. There is not much you can do at home if your cat is in shock--this needs to be treated with aggressive supportive care through intravenous fluids to maintain blood pressure, and medications to correct cardiac or respiratory issues.
Transmission fluid is a hydrocarbon and typically cause central nervous system signs. Other common products found in garages like brake fluid or ethylene glycol (anti-freeze). Ingestion results initially in neurological signs (in the first 30 minutes to 12 hours), followed by cardiac/respiratory issues (12-24 hours after ingestion), then kidney failure (24-72 hours after ingestion). Toxicity is somewhat dose dependent though small amounts are usually enough to be fatal without treatment.