The diurnal changes in proline and water contents of Zygophyllum quatarense and Francoeuria crispa were investigated under desert conditions. In both plants, the proline content was low before sunrise amounting to 10.42 μmoles/g fresh weight in Zygophyllum and 6.3 μmoles/g fresh weight in Francoeuria. By the progress of the day and the intensification of the evaporative power of the atmosphere, the proline content increased considerably in the two plants. The proline content reached maximal values at 5 p.m., reaching 23.36 μmoles/g in Zygophyllum and 11.16 μmoles/g in Francoeuria. The proline content of the artificially watered plants was kept at a low level throughout the day. The conditions and role of proline accumulation were discussed.