Laboratory experiments were conducted to study and compare nitrogen transformations occurring under both aerobic digestion and aerobic/anoxic (A/A) digestion. The process performance was examined at different sludge residence times (SRTs), temperatures and anoxic cycles. Both modes of operation gave comparable solids reduction results. However, introduction of anoxic periods to aerobic sludge digestion appears to be a promising alternative to control pH during digestion through endogenous nitrate respiration (ENR). Operating an aerobic digester with an anoxic phase to achieve complete denitrification would also improve supernatant quality over that achieved solely by aerobic digestion. Alternating A/A operation can conserve most of the influent alkalinity and maintain near neutral pH condition over prolonged periods. The A/A digestion of mixed primary/waste-activated sludge achieved up to 43.7% reductions in volatile suspended solids, 33.7% removal of total nitrogen, and a specific ENR rate of 5.75 x 10(-1) mg NO-N/mg VSSd. Optimum results were obtained at 10 d SRT, 30 degreesC temperature, and 50% anoxic cycle length.