This study was conducted in 4 villages in India during the hottest and driest season of the year to determine whether exclusively breast-fed infants need additional water under extremely hot and dry climatic conditions. The ambient temperature was 35-40 degrees C and the relative humidity 10-35%, except during the early morning hours. 63 urine samples were collected from 31 infants below 6 months of age and 28 samples from 13 infants aged 6-10 months, all of whom were receiving nothing but breast milk. Specific gravity (and corresponding osmolality) of urine samples from the younger group ranged from 1.004 (66 mosmol/litre) to 1.036 (1234 mosmol/litre), with a mean of 1.011 (322 mosmol/litre). For the older group the range was 1.005 (103 mosmol/litre) to 1.029 (978 mosmol/litre) and the mean was 1.015 (468 mosmol/litre). These values are well below levels of urine concentrations known to be attainable by infants of corresponding ages. Thus, even under hotter and drier climatic conditions than have previously been studied, healthy exclusively breast-fed infants do not require additional water. Exclusive breast feeding for the first 4-6 months is therefore a reasonable public health recommendation; it is imperative in areas where contaminated drinking water may contribute to infant morbidity, diarrhoeal disease in particular.