The temperate sea anemone Anemonia viridis (Forskal) forms an endosymbiotic association with dinoflagellate algae commonly referred to as zooxanthellae. It is now well established that under appropriate environmental conditions, these associations can be autotrophic for carbon. Under such conditions, many of these Symbioses, including A. viridis, not only retain excretory ammonium, but can take up ammonium added to the surrounding seawater. The flux from inorganic to organic nitrogen will be via the free amino acid pools and in A. viridis these were found to be markedly different between zooxanthellae and host with glycine and taurine dominant in the latter. When anemones were maintained with 20 muM ammonium, the concentration of free amino groups increased in the zooxanthellae but appeared not to change in the host. There was no evidence that the ratio of glutamine glutamate in zooxanthellae changed when anemones were maintained with 20 muM ammonium for 47 days. These ratios imply that zooxanthellae from this temperate symbiosis may not be nitrogen-limited. GDH was detected in both zooxanthellae and host where it was most active with the coenzyme NADPH. In addition, GDH showed activity when glutamine replaced ammonium as the substrate, indicating that (lie host may have alternative means to assimilate ammonium. Zooxanthellae were shown to possess GOGAT activity in the presence of a ferredoxin analogue. This suggests that in vivo zooxanthellae could assimilate ammonium via the activity of GS linked with ferredoxin-dependent GOGAT, Given evidence from other studies of rapid ammonium assimilation and essential amino acid synthesis in symbiotic host tissue, it appears that the capacity of cnidarians to metabolise nitrogen may at present be underestimated.