Perinatal development of cytochrome-c oxidase (complex IV) and ubiquinol-cytochrome-c reductase (complex III) was investigated in rat heart and liver by analysing catalytic properties, protein amounts, and subunit isoforms during the transition from the fetal to the adult state. The total amounts of complexes from milligram quantities of tissue, and the portions of isoforms of complex IV, were quantified densitometrically after isolation of the native complexes by blue native polyacrylamide gel electrophoresis and separation of the protein subunits by Tricine/SDS/PAGE [Schägger, H. & von Jagow, G. (1991) Anal. Biochem. 199, 223-231]. A parallel increase of protein amounts and catalytic activities during perinatal development was observed in heart and liver for complex III, but only in liver for complex IV. In heart, both a doubling of the turnover number of complex IV and a lowered Km for cytochrome c were observed. The altered enzymic properties correlated with the increase of heart type subunits VIa and VIII. The fetal enzymes from heart and liver seem to be identical to the adult liver isoform, as deduced from their enzymic properties and identical aminoterminal sequences of subunits VIa and VIII.