Marsupenaeus japonicus (11.47+/-0.71 g) exposed individually to six different nitrite and nitrate regimes {(0.002 (control), 0.359 and 1.456 mM nitrite combined with 0.005 (control) and 7.458 mM nitrate)} in salinity of 30 ppt (parts per thousand) were examined for hemocyanin oxygen affinity, the fractionation of oxyhemocyanin and deoxyhemocyanin, and the acid-base balance after 24 h. Ambient nitrite at concentration of 0.359 mM caused reduction of oxyhemocyanin and protein by 27 and 11%, respectively, whereas ambient nitrate as high as 7.458 mM caused reduction of oxyhemocyanin and protein by 10 and 7%. Ambient nitrite at concentration of 1.456 mM caused increases of P 5 0 (indicating reduced oxygen affinity) and pO 2 , but caused reduction in hemolymph pCO 2 , pH, HCO 3 - and TCO 2 . Following exposure to combined solutions of 1.456 mM nitrite and 7.458 mM nitrate there were no further changes in oxyhemocyanin, protein, hemolymph P 5 0 , pO 2 , pCO 2 , HCO 3 - and TCO 2 , but there was a significant reduction of pH.