OBJECTIVE: To evaluate motherś knowledge about oral rehydration therapy and to determine the concentration of sodium in sugar salt solutions prepared by the interviewed mothers. METHODS: Cross-sectional study of low-income families from the suburbs of Natal, a city in the state of Rio Grande do Norte. One hundred and thirty six households with children younger than 5 years were visited. The information on when to use the solution, how to administrate it and how to prepare it was provided by the interviewed mothers. The concentration of sodium was determined in 100 solutions prepared by those mothers. RESULTS: Only 9.0% of mothers had appropriate knowledge about when to use the solution and 21.0% knew how to administer it correctly. The error rates regarding the measurement of ingredients were: 2.5% for salt, 22.2% for water and 43.2% for sugar, according to the answers given by 81 mothers, who used a double-ended spoon made of plastic to measure quantities. In fifty percent of the analyzed samples, sodium was within safe limits (30 mmol/l to 100 mmol/l). Those considered dangerous (>100 mmol/l) or inefficient solutions (<30 mmol/l) reached the percentage of 47.0% and 3.0%, respectively. Out of the 69 solutions prepared with the double-ended spoon, 63.8% were found appropriate in terms of sodium concentration, while only 19.3% of the 31 solutions prepared using other measuring utensils were within acceptable limits (P=0.0001). CONCLUSION: Motherś knowledge about oral rehydration therapy was considered poor. Moreover, the concentration of sodium in sugar salt solutions seldom remains within safe limits, particularly those solutions in which the double-ended spoon was not used for preparation. These conclusions should be considered in future educational programs on oral rehydration.