Dark respiration of the symbiotic sea anemone Anemonia viridis (Forsk/il) was observed to increase by ~ 34% when anemones were exposed to hyperoxic sea water (150% oxygen saturation) overnight, and by 39% after exposure to 6 h in the light at a saturating irradiance of 300 btEm -2 s-1 at normoxia (100% oxygen saturation). No increase due to light stimulation was observed in aposymbiotic control anemones. In darkness, the oxygen concentration of the coelenteric fluid was hypoxic. However, within 10 min of anemones being illuminated, coelenteric fluid was hyperoxic, and it remained elevated throughout a 12 h light period. When measured over a 24 h period (12 h light: 12 h dark), the dark respiration rate increased gradually over the first 6 h of the light period until it was 35% above the dark night-time resting rate. It remained elevated throughout the remaining light period and for 2 h into the following dark period, after which it fell back to the resting rate. Gross photosynthesis (Pg .... ) increased significantly when anemones were exposed to either hyperoxia (150% oxygen saturation) or 300 gE m -2 s-~ at normoxia. This increase was not observed when symbiotic anemones were illuminated at a lowlight intensity of 100 gEm -2 s-a. The results of this study suggest that respiration in the dark is limited by oxygen diffusion and that normal respiration is restored in the daytime by utilisation of the oxygen released by photosynthesis. Furthermore, it appears that the increased respiration following exposure to high-light intensities provides a CO2-rich intracellular environment which further enhances the photosynthetic rate of the zooxanthellae.