Travelling with a mouse is an exercise in respecting scale. A mouse is light enough to be thrown by a sharp brake, narrow enough in the skull to pass any gap its head fits through, and guided so completely by smell that a carrier full of strange, scented bedding is a stressful place before the engine even starts. The recipe is a small solid carrier with fine ventilation, deep unscented bedding from home, cheap cardboard cover to hide in, and, for social mice, the companions they live with loaded in together so the group smells and sounds stay constant throughout the drive.
A carrier checked like a sieve
Inspect the carrier the way the mouse will: vent by vent and seam by seam, looking for any opening wider than a pencil. Solid plastic with a firmly latching lid beats bars unless the bar spacing is genuinely mouse-proof, and the lid deserves a second press at every corner because a mouse's first journey work is testing it from underneath. Fill the base deep with the colony's own bedding, add a toilet-roll tube and a small cardboard box as cover, and scatter their seed mix through the bedding so foraging fills the quiet stretches. A water bottle can ride clipped outside, tested for drips before you leave.
Groups, and the male exception
Female mice and neutered companions live socially and travel best as the group they are, piling together in the bedding as they do at home; separating one mouse for a routine trip adds loneliness to motion. A male mouse is the exception, living alone and travelling alone, and for him the familiar bedding and a worn piece of his own nest material do the reassuring. Coax each mouse into a short cardboard tube with a seed at the far end and lift the tube across, never by the tail, which is delicate and easily injured, and count heads before and after every transfer, because a missing mouse announces itself to no one.
A quiet, neutral-smelling cabin
Keep the carrier covered with a light cloth, out of sun, and away from air fresheners, food smells and the family dog's hopeful attention on the back seat. Mice read the world through their noses, and a cabin thick with perfume or fast-food scent is genuinely harder on them than the motion is. Belt the carrier so it cannot slide, set it on the floor or seat away from speaker vibration, and keep voices moderate. Short journeys need no intervention at all; on longer ones, stop to check the bottle has not soaked the nest and that everyone is accounted for and moving.
At the destination
Open the carrier inside the cage or travel enclosure rather than tipping mice out into the open, and let them scatter into their bedding under their own power. Fresh water, scattered food and a hands-off few hours restore normal behaviour: exploring, grooming each other and rebuilding the nest. A mouse that sits hunched and ruffled, breathes audibly or has not eaten by the following day should be raised with a vet, since mice hide illness well and small bodies have small reserves.
The detail worth writing down
Colonies change, weights are hard to judge by eye at this size, and a vet meeting six similar mice benefits from knowing which one is which and what each normally eats. Keeping the colony's individuals, foods and past visit findings noted in KinsLe's Pet Passport gives that clinic a fighting chance of treating the right mouse with the right history in front of them.