The response of the free amino acid pools of two nereid polychaetes, Neanthes succinea and Leonereis culveri to both increased and decreased salinities was examined. In both species, glycine and alanine accounted for most of the observed change in the total free amino acid (FAA) pool size. Glutamine values increased under hypo-osmotic conditions suggesting that this amino acid plays a role in nitrogen storage during the reduction in FAAs which occurs with reduced salinity. The salinity response was superimposed on a 'laboratory stress' response characterized by a generalized increase in the FAA pool size. The two responses appeared to be additive in nature. Significant differences in the responses of individual amino acids between these two species indicates that interspecific variation exists in the response to salinity change in nereid polychaetes.