Human interaction causes stress to (non-pet) laboratory mice. Handling should be as fast as possible and as limited as possible, and cages should be enriched as much as possible. Source: I work with mice in tumor immunology. Rats, however, can really be very difficult to work with though. Because they clearly have a much higher social awareness and are much more intelligent then mice. You can kill a mouse next to his buddy, and he couldn't care less. You kill a rat across the room to a random rat, the guy/gall starts screaming the ears of your head. Source: I don't (want to) work with rats.