Sun and shade samples of two aquatic bryophytes, the moss Fontinalis antipyretica and the liverwort Jungermannia exsertifolia subsp. cordifolia, were collected from a mountain stream and cultivated in the laboratory under two artificially imposed radiation regimes: control (only photosynthetically active radiation, PAR) and UV-B (PAR UV-B). Samples were cultured at 2°C for 78 hr under continuous radiation to determine whether the physiological responses of the two bryophytes to UV-B radiation depended on their previous field acclimation to sun or shade conditions. We also aimed to study whether the short-term effects of UV-B were similar to those caused by longer exposure. Fontinalis antipyretica was more sensitive to UV-B treatment than Jungermannia exsertifolia subsp. cordifolia, showing significant decreases in several physiological variables indicative of vitality: PN rates, OD430/OD410, OD665/OD665a and especially Fv/ Fm. This higher sensitivity occurred in both sun and shade samples of the moss. These results reproduce the differences between both species that were found in previous more prolonged (36– 82 days) experiments. These types of short-term tests may therefore be used instead of long-duration tests to evaluate the UV-B tolerance of bryophytes. Shade samples were more sensitive to UV-B treatment than sun samples, but only in the more UV-B-sensitive species (Fontinalis antipyretica). Fv/Fm was the physiological variable that better discriminated both types of samples in the moss, since it decreased 42% in the shade samples and only 27% in the sun samples at the end of the culture period. In Jungermannia exsertifolia subsp. cordifolia, controls and UV-B-treated samples were not significantly different in either the sun or the shade samples. Principal Components Analyses (PCA) for each species, ranking the physiological results along the culture period for each radiation regime and field exposure strongly supported these conclusions and also showed that the loading factors of PCAs may be helpful for establishing the combination of key variables responsible for the differences between controls and UV-B-treated samples, or between UV-B-treated sun and shade samples.