Studies with whole cells and mitochondrial fractions revealed increased respiratory activity in Aspergillus repens grown under salt stress conditions. The state 3 and state 4 respiration rates, P : 0 ratios, and Mg2+-dependent ATPase were higher in mitochondria of stressed cells than in control cells. A. repens respires via an antimycin A- and cyanide-sensitive pathway. Oligomycin, dicyclohexylcarbodiimide (DCCD) and rotenone inhibited respiration rates less in mitochondria of stressed cells than in controls. Though 2,4-dinitrophenol (DNP), carbonyl cyanide-m-chlorophenylhydrazone (m-Cl-CCP), and carbonyl cyanide-p-trifluoro-methylhydrazone (p-F3-CCP) did not stimulate Mg2+-ATPase activity, DNP enhanced the respiration rates, whereas m-Cl-CCP and p-F3-CCP decreased the respiration rates in either condition; mitochondria of stressed cells exhibited a lower degree of inhibition than controls. Addition of DNP, oligomycin, and DCCD inhibited the basal Mg2+-ATPase (ATPase activity without Mg2+ addition). Oligomycin inhibited the Mg2+ -ATPase. DCCD showed less inhibition in mitochondria under stress than did the controls. Levels of some respiratory enzymes were higher in the culture grown under stress than in the controls.