The partition test was used to estimate behavioural reactivity of male mice to conspecifics. Paired males were placed in the neighbouring compartments of a common cage separated into halves by a transparent partition with holes permitting animals to see and smell each other but preventing physical contact. Winners (with long experience of victories in daily inter-male confrontations) approached the partition and spent longer near it reacting to the partner in the neighbouring compartment than losers with long experience of defeat. Five defeats of winners in agonistic confrontations with stronger aggressive individuals led to a decrease of behavioural activity near the partition. Losers did not notice unfamiliar males in the next compartment. Under such conditions, winners or intact males significantly increased parameters of behavioural reactivity near partition to an unfamiliar neighbour. The data obtained are discussed in terms of decrease of the loser's behavioral reactivity to conspecifics as a result of the experience of defeat in social confrontations leading to the development of anxiety or, possibly, of failure of olfactory perception.