After repeated gentle handling they can be trained to receive some noxious stimuli such as injections with minimal restraint. Males are more aggressive than females. Rats will bite without warning but not repeatedly. Animals of the same sex may be housed together and when a male is placed with a female it will attempt to mate immediately, although the female will accept the male only in early oestrus. A vaginal plug, which is a mixture of secretions of the vesicular and coagulating glands of the male, fills the vagina from the cervix to the vulva after mating.
Plugs remain for several hours and then fall out and are usually seen on the bottom of the cage.
Observation of a vaginal plug is taken as the first day of pregnancy. Breeding rats are housed either as monogamous pairs or in trios of 1 male to 2 females or harems of 1 male to 6 females. Sexual maturity is reached by 8 weeks but some inbred strains take up to 12-16 weeks. In polygamous systems females are removed from mating boxes before parturition, as undisturbed females produce more milk and larger young and exhibit better mothering behaviour than if left with other females. At birth the young are hairless, blind and toothless.