Aiming to clarify the mechanism of inhibition of (Na super(+), K super(+))-ATPase activity by polyamines, we examined the effects of exogenous putrescine, spermidine, and spermine on the kinetic behavior of phosphoenzyme-linked partial reactions using a microsomal gill (Na super(+), K super(+))-ATPase from juvenile and adult M. amazonicum, a freshwater palaemonid shrimp. The time course of phosphointermediate formation is greater (0.089 plus or minus 0.006 s super(-1)) in adults than in juveniles (0.053 plus or minus 0.003 s super(-1)) for spermidine, but similar to juveniles (0.059 plus or minus 0.004 s super(-1)) for putrescine. Maximum phosphointermediate formation for the (Na super(+), K super(+))-ATPase from juveniles decreased by 46% and 32% with spermidine and putrescine, respectively. In adults, maximum phosphointermediate levels decreased by 50% and 8%, respectively. For both spermidine and putrescine, dephosphorylation rates were higher for adults than for juveniles, and were higher than in controls without polyamines. Spermine had a negligible effect (<10%) on phosphorylation/dephosphorylation rates of both juvenile and adult enzymes. This is the first report on the effects of polyamines on phosphoenzyme-linked partial reactions in juvenile and adult M. amazonicum gill (Na super(+), K super(+))-ATPases. Our findings suggest that the phosphorylation/dephosphorylation steps of this gill enzyme may be regulated by polyamines during ontogenetic development.