Human activity can be seen as an intentional, context-dependent enterprise explained through meanings the actors attach to their activity and their directly observable interactions with the environment. The authors have demonstrated previously a new conceptual framework to describe the anaesthetist's activity. One of the central concepts besides orientation is habit of action referring to the way in which the actor has organized his actions when interacting with his environment, in this case a patient with unique physiological potentials, information monitors and anaesthetic drugs. The activity dependent on contingent, particular circumstances, needs to be studied as it appears in a natural situation. Using an idiographic study design the authors have examined the activity of eight expert anaesthetists in clinical settings to determine the characteristics of their habits of action. To capture the fleeting circumstances during the anaesthetic process, a wide observational basis was necessary. It consisted of videotapes, detailed expert observations, and interviews. The conceptual analysis of the subject, habit of action, is described step-by-step. Two distinct habits of action could be identified, confirming earlier results. The interpretative habit of action was characterized by extensive use of situational information in order to construct a cumulative conception of the patient's physiological potentials to control the process accurately. Moreover, rich dialogue between formal professional knowledge and patient-specific, particular knowledge was evident. The reactive habit of action was characterized by a tendency to regulate the process by means of predetermined conventional ranges of measured patient parameters shown by monitors. The authors discuss their methodological solutions and results, and explicate their differences to the earlier approaches.