The aim of this work was to identify behavioural differences between the domesticated ferretMustela furoand its wild counterpart the European polecatM. putoriusand to obtain objective measures of one or more of these differences.The animals used in this study were tameM. putorius, M. furo, wild caught hybrids and laboratory reared F1hybrids between the two species.Ferrets differed in their behaviour in unfamiliar environments and in the fact that they do not develop fear of man. The F1hybrids resembledM. putoriusin that they develop a fear of man if left with their mother during a critical period between 7½ and 8½ weeks of age. The phenomenon of imprinting may be involved in this developmental process.The attention response to a rustling noise was investigated under controlled conditions revealing differences in response decrement between the four types of polecat.M. putoriusand the hybrids habituated more rapidly to the sound than did the ferrets.The results obtained for F1hybrids depended upon the previous environmental history of the individual. Those individuals which had been living in outdoor cages responded differently from the ones in indoor cages. The hypothesis was put forward that the hybrids respond more frequently if the experimental situation provides a greater contrast with their normal environment and that the individual's response threshold is therefore “set” to specific environmental conditions.The results obtained agree with the hypothesis first put forward by Lorenz (1953) that the behaviour of domesticated animals resembles that of juvenile individuals of their wild counterpart.Summary:A comparison is made between the behaviour of European polecats (Mustela putorius), the domesticated ferret (M. furo) and hybrids between the two species.Differences were found in exploratory behaviour, fear of man and the frequency of occurrence of the “attention response”.The occurrence of the “attention response” was studied under controlled conditions. It was found, on playing the rustling sound 40 times at minute intervals, that this sound elicited the attention response for longer in ferrets than in European polecats and hybrids between the two species. The most rapid habituation occurred in polecats, the F1hybrids being intermediate in their response betweenM. putoriusandM. furo.The results obtained for the F1hybrids were influenced by the conditions under which the animals were previously caged.The results appear to support Lorenz' view that domesticated animals show juvenility in their behaviour as compared with their wild