If two cats who used to get along suddenly exhibit mild aggression, place the two cats in separate rooms for a few days or weeks. Reward each cat's behavior when they interact with each other well. Provide the cats with identical beds, bowls and litter boxes in various areas.
Placing bickering cats in different rooms for an extended amount of time allows the cats to smell and hear each other without interacting. Switch the cats' rooms each day so that each cat can grow accustomed to the other’s scent. After several days, crack the door open slightly. If the cats remain relaxed upon seeing each other, they are ready to be reunited. Separate the cats if either one starts hissing, growling, spitting or exhibiting other signs of aggression. Prolong the separation period if the cats are severely aggressive with each other, and perform daily reintroduction sessions that slowly move the cats closer to each other.
Rubbing tuna juice on the cats' bodies and heads and placing them in the same room typically causes them to start grooming themselves, which relaxes each cat and may encourage interaction. Behavioral medication can also help reduce aggressive and anxious behaviors in cats. Utilize a certified applied animal behaviorist or board-certified veterinary behaviorist to evaluate and provide behavior remedies.