With the aim of implementing an intervention program on physician's prescribing behaviour for diarrhoea in children under five, an ethnoepidemiological study was conducted in Pelotas (Brazil), from February to April 1993. Information on prescription of drugs was obtained through record review of 381 cases of diarrhoea provided by 33 medical doctors from eight health centres. Trained field workers observed a total of 54 clinical consultations due to diarrhoea. Brief exit interviews with the mothers were performed just after the observations. Twenty-seven open-ended home interviews were made with the mothers the day after they had been observed. After all observations had been completed, open-ended interviews were conducted with 21 physicians. The results showed that there is a misunderstanding of the role of ORS in the treatment of diarrhoea: mothers want something to "cut" diarrhoea and they notice that ORS does not act in this way and doctors do not explain the action of ORS in diarrhoea management. Comparing with record reviews, during observations a child had a lower probability of receiving an antibiotic or antidiarrheal drug prescription. This finding indicates that other variables than technical skills are involved in doctor's prescribing behaviour. A lack of ability or of motivation to deal with "anxious or difficult mothers" led some doctors to enhance antibiotic or other non-recommended drugs to manage diarrhoea. Therefore, efforts to improve the quality of case management of diarrhoea, through intervention programmes in the government health sector, are needed in Pelotas.