IT has often been said that the inhabitants of arid regions seem to require far less fluid than non-indigenous visitors1, but opinions vary as to whether this discrepancy is attributable to ethnic differences in the inherent physiological mechanisms which are concerned with sweating and water metabolism, or to cultural-adaptive factors such as diet, clothing and mode of activity2–4. In an attempt to elucidate this problem, I and a Toureg nomad undertook a 500 mile journey in the Central Sahara, and I present here the observations which were made of our respective fluid requirements during this trek.