Uptake and loss of water by six lichen species from the Argentine Islands, Antarctic Peninsula, were studied in their natural habitat andin laboratory studies. Under field conditions, during a period of rain, uptake of moisture ranged from 15% d w h{sup}-1{end} for Usnea antarctica to almost 90% for Mastodia tesselata. Loss rates after the rain ceased were lower than the uptake rates and ranged from 8.5% d w h{sup}-1{end} in Umbilicaria decussata to 38.8% in M. tesselata. A comparison of thalli of M. tesselata from the shoreline with thalli collected further inland showed significant differences in maximum watercontent and in rates of water loss and uptake between thalli from these sites, which could be ascribed to the presence of salt in the thalli of M. tesselata from the shore. Thallus samples of Usnea antarctica collected from an exposed site showed lower uptake rates and higher loss rates of water than samples collected from a more sheltered site. Under laboratory conditions the maximum moisture content ranged from 67% in Usnea antarctica to 391% in M. tesselata. Exposed to an atmosphere of c. 40% relative humidity and c. 15 deg.C, lichen thalli lost their water during the first hour of the experiment at rates ranging from 34% of the maximum moisture content in Umbilicaria propagulifera to 70% in U. decussata. Rates of water loss diminished with time, and after 4--6 hours equilibrium was reached, with water contents of 14--16% of thallus dry weight. The microclimate is the driving force for the thallus water regime. Specific differences in rates of water loss and water uptake depend on the morphology and anatomy of the lichen thalli. The results of the laboratory experiments agree with those from the field study. However, local differences in microclimatic or other environmental factors can be responsible for significant differences in the water regime of thalli of the same species, a result which can only be obtained from field studies. [ABSTRACT FROM AUTHOR] more...