Mechanisms determining the ability of individuals to recognize their kin can be broadly divided into those prenatally determined and those postnatally determined. The role of both of these possibilities was examined in an investigation of sibling recognition in the rat (Rattus norvegicus). Rat pups were tested for their preference for ‘siblings’, (either littermates and genetic siblings—natural siblings; just littermates—any recognition due to postnatal factors; or just genetic siblings—any recognition due to prenatal factors) as opposed to unfamiliar age-mates between the ages of 6 and 22 days, and also for their ability to detect conspecific odour. The results indicate that rats can recognize their siblings (Experiment V) and this can be achieved solely postnatally (Experiments I, VI), or solely prenatally (Experiments II, VII). Other results demonstrated that rats, as early as day 2, and probably earlier, can detect and respond preferentially to conspecific odours (Experiments III, IV and VIII). The possibilities of how these mechanisms, both postnatal and prenatal, function in the acquisition of sibling recognition are discussed. This is the first study in which individuals separated from all genetic relations postnatally are shown to posses the ability to recognize kin.