Both C-13 nuclear magnetic resonance (NMR) spectroscopy and High Performance Anion Exchange (HPAE) chromatography were used to examine the alditols mannitol and volemitol found in the brown alga Pelvetia canaliculata (Linnaeus) Decaisne et Thuret (Phaeophycota, Fucales, Fucaceae). Carbohydrates were extracted using either ethanol or distilled water and our investigations have shown that both methods give qualitatively and quantitatively similar results. The ratios of alditols extracted from the seaweed under different growth conditions (temperature and emersion time) were compared. In the laboratory, growth rates of P. canaliculata increased with temperature up to 20 degrees C and decreased thereafter, but results indicated that the degree to which plants responded to temperature is seasonally dependent. Greater increases occurred during winter and spring, possibly due to low-temperature acclimation of field-collected plants in winter. Mannitol and volemitol concentrations in P. canaliculata increased in culture with temperatures up to 27 degrees C. Both mannitol and volemitol concentrations remained similar during continuous emersion for 7 d at 10 degrees C. By contrast, in plants emersed continuously for 8 or 12 d at 25 degrees C, volemitol concentrations decreased significantly, whilst mannitol concentrations remained similar. But when plants were emersed continuously for varying periods and subsequently re-immersed in seawater, recovery of photosynthesis was complete in plants which had been emersed at 10 degrees C for up to 7 d. When cultured at 25 degrees C, photosynthesis recovered only when plants had been emersed for no longer than 4 d. As optimum growth occured at higher temperatures than in other intertidal fucoids and P. canaliculata appeared well adapted to prolonged periods of emersion, even at high temperatures. The accumulation of mannitol and volemitol during growth at higher temperatures when submerged and a decrease during emersion at high temperatures may suggest a possible role for these carbohydrates as stress metabolites. However, further research is required to explain how the unique presence of volemitol in P. canaliculata may contribute to its ability to withstand unfavourable conditions in the intertidal zone during periods of extreme and prolonged dehydration.