Despite the biological and economic importance of the freshwater shrimp Macrobrachium amazonicum, a role for biogenic amines in the osmoregulatory capability of this diadromous species remains unstudied. This investigation provides an extensive kinetic characterization of the effects of the exogenous polyamines spermine, spermidine, and putrescine on (Na+, K+)-ATPase activity in gill microsomal preparations from juvenile and adult shrimps at varying concentrations of ATP, Mg2+, Na+, and K+, and on inhibition by ouabain. (Na+, K+)-ATPase activity in both juvenile and adult shrimps is inhibited by spermidine (60–95%) and putrescine (40–70%) while spermine has a negligible inhibitory effect (<10%). Putrescine affects the kinetics of ATP hydrolysis and enzyme affinity (KM) for ATP in both enzymes, while spermidine alters specific activity only. Spermidine increases Mg2+ affinity of the adult (Na+, K+)-ATPase while putrescine decreases Mg2+ affinity in both enzymes. Spermidine decreases Na+ affinity by ≈50% in both enzymes, while putrescine increases affinity of the juvenile enzyme twofold. These biogenic amines cause slight changes in KM values in both enzymes. These findings suggest that the effect of biogenic amines on gill (Na+, K+)-ATPase activity and kinetics may be species and stage specific.