In this paper the effects of the ammonia concentration on AR are explained. It may be expected that NH4+ concentration will have a negative influence on AR, where higher concentration pro- duces lower AR; however, there is no clear evidence and the results depend on the membrane used. For recirculation aquaculture sys- tems (RAS) this behaviour becomes interesting, as the ammonia concentration increases over the time and the concentration limit for use with membrane processes must be identified before any investment in the new technologies and applications can be made. Thus, the aim of this paper was to analyze the capacity to retain ammonia by NF and RO membranes in a non steady state sys- tem, where the concentration of ammonia is increasing during the time, at pH and concentration conditions interesting for aquacul- ture applications.