BACKGROUND: The aim of this article was to anthropologically analyse knowledge and practices about hydration and rehydration in a specific ethnographic context, where diverse therapies are combined to treat and take care of child diarrhoea as part of a wider social process that circumscribes transactions between self-care and biomedicine. METHODS: Ethnographic data from a qualitative study in the neighbourhood of Nova Constituinte (Salvador, Bahia) which was part of an interdisciplinary project aimed at epidemiologically evaluating an environmental sanitation programme. These data results from a series of in-depth interviews of 29 interviewees and field observations collected over two stages (1997/1998-2003/2004). RESULTS: Knowledge about hydration and rehydration is practical knowledge that demonstrates some of the cultural limits of dehydration in terms of the normality or pathology criteria related to child diarrhoea. This knowledge belongs to local interpretations, treatment experiences and the care that mothers provide in relation to their child’s diarrhoea. We observed a process of medicalisation in the discourse about hydration and self-care. CONCLUSIONS: Unlike rehydration, hydration is structural to self-care processes. While the former constitutes a way of alleviating diarrhoea, the latter is a type of care centred on healing. The difference between these practices does not lie in the type of remedies used but in the meaning attributed to them and the way they are combined.