The playful behaviour of laboratory rats (Rattus norvegicus) was investigated in litters of five individuals with the mother present; parallel observations were made on mice (Mus musculus). Seven mixed litters containing four young rats and a young mouse fostered at birth were also observed.

Solitary play was recorded in both species and took a similar form but social play was only observed in rats. In rats, solitary play frequently preceeded social play.

The behavioural elements involved in the social play of Rattus norvegicus were described, and the majority of these were the playful equivalent of adult agonistic behaviour elements. These social play elements were found to be organized into definite sequences which differed from those of adult aggression. Each behavioural element was found to act as a social releaser.

Young mice did not respond playfully to social play from a rat litter mate; mice were less attractive to rats as playmates in comparison with fellow rats.