Why rats usually make this easy — with the right stages
Rats are intensely social and most introductions succeed, but a resident rat can still be territorial about its cage, and putting a stranger straight into that cage is the classic mistake. Work in stages that move from smell, to meetings, to shared space. Keep the newcomer quarantined in a separate cage first if it has come from a rescue or shop with unknown health history, and always introduce on ground that belongs to neither rat.
Scent groundwork before the first meeting
Before the bathtub stage, let the rats trade smells: house their cages in the same room, and swap a hammock or a handful of bedding between cages every couple of days. By the first meeting, each rat already carries some of the other's scent, and you will usually see calmer sniffing rather than alarm. Age shapes expectations too — kittens are typically accepted within a session or two, while older males may need a week of repeated neutral meetings before the carrier stage. None of that is failure; it is the process working at the speed those particular rats require.
Stage one: meeting on neutral ground
A dry bathtub or a large box neither rat has played in makes a good first meeting place. Scatter treats and a shallow dish of something soft and tasty that they must eat side by side, add a couple of hides with two exits, and keep the session to fifteen or twenty minutes. Expect sniffing, following, some squeaking and possibly a rat standing over the other; forced grooming and pinning are normal hierarchy behaviour. Puffed fur, sidling up sideways, hissing or bites that draw blood mean end the session and repeat neutral meetings for a few more days.
Stage two: the small-space step
Once neutral meetings are calm, use close quarters to consolidate: a small carrier or small plain cage with only bedding, food scattered rather than in a bowl, and water. Rats in a snug, boring space tend to pile up and sleep together, and waking up in a warm heap does more for a bond than hours of open-floor negotiation. Keep them in the small space for several hours or overnight with supervision nearby, then move to a slightly larger bare cage if all stays peaceful, adding hammocks and toys gradually as each stage holds.
Stage three: the main cage and monitoring
The permanent cage should be scrubbed, rearranged and neutral when the pair move in, so it belongs to both. Watch the first days for one rat guarding food or excluding the other from sleeping spots, and weigh both weekly at first, since stress and bullying show up in weight before anything else. Kittens usually integrate fastest, adult males slowest, but the staged method works across ages. A rat wounded in a fight, or one who becomes withdrawn and stops eating, should see a vet. Listing each introduction stage in KinsLe's Care Teams means the next step — another neutral session, the carrier stage, the cage move — is visible to whoever is home, so the process keeps its momentum day by day.