My female cat growls at the new adopted male cat. What can I do?

That actually sounds like very normal behavior for a cat who is adjusting to a major change. It sounds like Arlo is working on getting comfortable in his new environment but isn't so sure about the humans there yet. I would recommend giving him lots of space, not taking the growling personally, and giving him time (probably a week or two, maybe even months) to warm up. Let him be the one who chooses to interact with you, not the other way around. you can definitely use food and offerings of toys to try to entice him to come interact with you, but it should still be his choice. Congratulations on the new addition, I hope he settles in soon!

Most cats will settle down within a few weeks. Kittens do play rough and it can be hard to tell. If they willingly come together, play willingly and you don't see any bullying behavior (avoidance, hiding, arching the back and puffing up the tail) they are likely playing rough. If they play too rough they could injure each other by accident so continue to separate them when not supervised but most kittens will not require this. Giving each kitten one on one play time with you to burn energy, feeding out of a food puzzle and providing cats trees/other environmental stimulation can also help. Continue the current therapy as well. I hope this helps.

The first thing I would do would be to rule out a medical condition. Conditions such as hyperthyroidism and urinary tract infections can sometimes manifest as aggression. If your veterinarians determines that everything is normal, I would recommend a product called Feliway which can be used to calm cats. It can be purchased over the counter at any pet store or online.

If it has only been a few days since moving into the new house, I would give them some time to adjust. It can take up to two weeks for animals to feel comfortable in a new home. If after another week they are not adjusting I would talk with your veterinarian about anxiety medication for them. I would also make sure both cats are getting a lot of attention, play time, have enough litter boxes and have places they feel comfortable in. Thank you for using Petco Pet Education Center, formerly Petcoach.

Unfortunately it's very common. It means you need to rewind a bit and introducing more slowly. It can take months to fully integrate a new cat (and sometimes it doesn't even work at all). If eating in the same room was fine, go back to that for a few weeks, but no free roaming.

Do it gradually. First, allow them to hear and smell one another but not see each other. This is best accomplished by keeping her in a small, closed room. Once your male has accepted her presence, have them see one another form a distance for ever increasing periods of time. The next step would be to place her in a closed carrier in the same room as your male. The final step step would be a supervised physical introduction. Use leashes on both for safety. The entire process may take days to weeks. A pheromone product, such as Feliway Spray or plug in diffusers, may decrease any anxiety or aggression. Ideally, you will also need to provide a litterbox for each cat plus an additional box in a neutral area. Extra bowls, toys and beds should also be provided to prevent resource protection.

Introducing a new cat to the household can be quite disruptive. You will likely have to go back to the beginning, and give them time apart for several weeks. This will mean keeping the new male cat in a separate part of the house, where he can't be around your female cat. You don't even want them to see each other at this point, but it is ok if they are able to smell each other under the door. After a few weeks of this, try letting them see each other, but not in the same room. Finally, you can let them be in the same room, but make sure they are supervised and make sure that each cat has a way to get away from the other. I would also suggest that there are more than enough cats in the world - too many, in fact - and I recommend you considering getting both of them altered, instead of trying to force them to breed.