ABSTRACT: The bioavailibility of dissolved organic nitrogen (DON) in river water entering estuaries was examined for the Delaware and Hudson Rivers, USA. Water collected from above the salinity intrusion zone of each river was filtered, brought to a salinity of 15ppt, and inoculated with estuarine bacteria. Bacterial production rates (8 to 26 x 10⁵ cells ml^-1 d^-1) during the initial 2 d in these experiments were within the range measured in these and other estuaries, indicatingthat riverine dissolved organic matter can contribute to production of estuarine bacteria. Average DON concentrations decreased by 40 to 72% within the 10 to 15 d time course of the experiments; the decreases in DON were accounted for by increases inmicrobial biomass plus remineralization to inorganic nitrogen. The time scale over which DON was utilized suggests that in estuaries with residence times on the order of weeks to months, such as Delaware Bay, river inputs of the biologically availableportion of DON are first utilized within the estuary. In contrast, in estuaries with residences times of less than a week, such as New York Bay, a portion of the biologically available DON may be utilized first within the estuary, with the remainderexported and utilized in continental shelf waters. The large proportion of the DON that was biologically available in these experiments, coupled with the knowledge that inputs of organic nitrogen can account for 20 to 90% of the total nitrogen loading toestuaries, suggests that organic nitrogen inputs may contribute more to estuarine and shelf eutrophication than was previously suspected. These experiments demonstrate that dissolved inorganic nitrogen (DIN) inputs underestimate, and total nitrogen inputslikely overestimate, the inputs of biologically available N to estuaries. In order to develop a 'biologically available nitrogen budget' for an ecosystem, DIN inputs, plus that portion of the organic N that is biologically available must be quantified.