The effectiveness of probabilistic and ethological approaches to the analysis of normal and pathological animal behaviour is shown. The probabilistic analysis of the frequency, duration and sequence of a large variety of behavioural acts and postures permits to single out the most significant events and diad transitions in the interaction between two animals (previously isolated or grouped). Computer and probabilistic analysis reveals sharp differences in the structure of these acts and their interdependence in isolated and in grouped mice. It is noted that in isolation there occur fixation of the program of aggressive behaviour, a sharp narrowing of the spectrum of possible transitions from aggression to other forms of intraspecific behaviour, aggression being little controlled by intraspecific inhibitory factors which is regarded as pathology of behaviour.