Nitrite　and　nitrate　have　been　generally　recognized　to　have　an　inhibitory　effect　on　metabolism　of　denitrifiers.　The　nitrite　inhibition　on　nitrate　reduction　under　various　pH　conditions　and　the　cross　effect　of　the　both　electron　acceptors　on　each　other　were　investigated　through　a　series　of　batch　tests　by　using　a　biological　nutrient　removal　(BNR)　sludge.　The　results　showed　that　the　nitrate　reduction　activity　had　a　much　stronger　relationship　with　the　free　nitrous　acid　(FNA)　than　that　of　nitrite　concentration,　suggesting　that　FNA,　rather　than　nitrite　is　likely　the　actual　inhibitor　on　nitrate　reduction.　Sixty　percent　inhibition　of　nitrate　reduction　was　observed　at　an　FNA　concentration　of　0.01-0.025　mg　·　L-1,　while　total　inhibition　occurred　when　the　FNA　concentration　was　greater　than　0.2　mg　·　L-1.　Furthermore,　nitrite　reduction　by　the　BNR　sludge　was　also　found　to　be　inhibited　by　HNO2.　The　reduction　rate　decreased　by　approximately　80%　when　the　FNA　concentration　was　increased　from　0.01　mg　·　L-1　to　0.2　mg　·　L-1.　The　inhibitory　effect　of　nitrate　on　nitrite　reduction　was　found　to　be　insignificant,　with　the　most　recovery　rates　under　different　nitrate　concentrations　larger　than　90%.　However,　the　nitrate　reduction　rate　was　observed　to　recover　only　3.04%-72.54%.　The　recovery　rate　from　inhibition　was　independent　of　the　duration　of　the　inhibition　and　the　feeding　mode　of　inhibitor,　but　strongly　dependent　on　the　concentration　of　inhibitor　the　biomass　was　exposed　to　during　the　inhibition　period.