The burrows of macroinfauna are significant sites of sediment-water nitrogen exchange and associated microbial activity. In this study, the exchange of dissolved inorganic nitrogen (DIN) and nitrogen cycle reaction rates were quantified in individual burrows of the estuarine polychaete Nereis virens. Burrow ventilation rate and DIN (NH4+, NO2, NO3 and N2O) exchange were determined at 22°C in individual, inhabited burrows with and without the presence of C2H2 (an NH4+ oxidation, N2O reduction block). Ventilation cycles were unaffected by C2H2, but worm metabolism (O2 uptake) and excretion of NH4+ were enhanced by ∼100% and ∼50%, respectively. Time-specific DIN exchange patterns were quantitatively modeled by relating burrow water concentration changes, excretion, and ventilation rates. The highest rates were at the start of ventilation periods and decreased or increased (depending on the solute) exponentially to a steady state level. The presence of C2H2 increased NH4+ release from burrows and changed the NO2 flux from a high release (∼300 nmol h−1) to an uptake (∼–30 nmol h−1). Nitrate uptake was independent of C2H2, presumably because overlying water NO3 concentration was high (∼100 μM). Indirect estimates of nitrification corresponded to the burrow release of NO2 without C2H2. Approximately half of the NO2 + NO3 uptake in burrows was due to denitrification. In microcosms with and without N. virens (875 m−2), denitrification was stimulated 3-fold by N. virens and the ratio denitrification/nitrification increased from 0.61 to 1.11. The changes in DIN flux and denitrification caused by N. virens corresponded well to the rates extrapolated from individual burrows to the appropriate worm density of 875 m−2. At the abundance used, N. virens burrows were responsible for 37% and 66% of the total sediment nitrification and denitrification, respectively.