An inbred strain of the polychaetous annelid Neanthes arenaceodentata was subjected to reduced dissolved oxygen and salinity levels under laboratory conditions and effects of these treatments on the free amino acid pool studied. Changes in the relative proportions of free amino acids were observed after exposure to lowered dissolved oxygen concentrations. It is hypothesized that these relate to changes in total protein levels and conversion to energy production under anaerobic conditions. The total free amino acid concentration, expressed as total optical density of ninhydrin positive spots, decreased at lowered salinities along with alteration in the relative proportions of amino acids. These changes probably relate to osmoregulation by the worm under reduced salinities.