Quarantine first. Keep new rats in a separate room, ideally with separate airflow, for two to three weeks, and wash your hands and change tops between groups. This protects against respiratory infections, which spread easily. Introduce on neutral territory: a bathtub, a table top, a playpen or a room neither group owns. Clean it beforehand so no one has scent-marked it. Scatter food so mouths are busy. Go slowly. Common approaches include swapping bedding between cages for a few days, then side-by-side cages, then short neutral meetings, then longer ones, then a fully cleaned cage with rearranged furniture and everything scrubbed of old scent. Remove hides and tunnels for the first shared nights so no one can be cornered. Expect noise. Squeaking, pinning, boxing and power grooming are normal. Step in for: sustained chasing with screaming, a rat who cannot escape, or blood. Have a towel or oven glove to hand and never put bare hands into a rat fight. Pausing and going back a step is normal, not failure. Introductions can take days or several weeks. Hardest matches are two adult unneutered males. Neutering, on veterinary advice, resolves a large share of those cases. Easiest are youngsters into an adult group.