Some general features of the respiratory chain and respiratory control were characterized in coupled mitochondrial preparations from Leishmania mexicana promastigotes. O2 uptake was sensitive to the electron-transfer inhibitors rotenone, flavone, malonate, 4,4,4-trifluoro-1-(2-thienyl) 1.3 butanedione (TTFA), antimycin A, 2n-nonyl-4-hydroxyquinoline- N-oxide (HQNO), myxothiazol, cyanide and azide. A high concentration of rotenone (60 mM) was required to inhibit O2 uptake effectively. Difference spectra revealed the presence of cytochromes (a a3), b and c. Respiratory control was stimulated 2-fold by ADP with different exogenous oxidizable substrates. Calculated ADP/O ratios were consistent with the notion that ascorbate/N,N,N%,N%-tetramethylphenylenediamine (TMPD)-linked and FAD-linked respiration proceeds, respectively, with one third and two thirds of the ATP producing capacity of NADH-linked respiration. State 3 was suppressed by the ATP synthase inhibitors oligomycin and aurovertin and by the adenine nucleotide translocator inhibitors atractyloside and carboxy atractyloside. The protonophore carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) provoked state 3u respiration. The mitochondrial preparation was capable of Ca2 uptake and Ca2 stimulated respiration. Data obtained suggests strongly that mitochondrial complexes I, II, III and IV are present in a major pathway of electron-transfer and that oxidative phosphorylation might proceed with high bioenergetic efficiency.Instituto de Biologı´a Experimental, Facultad de Ciencias, Uni6ersidad Central de Venezuela, Apartado 47114, Caracas, Venezuela.