The specific contents of cytochrome P-450scc and adrenodoxin in corpora lutea of late pregnant sheep were, respectively, 1/5 and 1/8 that of corpora lutea of the oestrous cycle, suggesting lower steroidogenic enzyme capacity in the former. The contents of Complex V proteins were also lower in the corpora lutea of late pregnancy. It was observed in the immunoblots of both Complex V and cytochrome P-450scc that immunoreactive bands of molecular weights lower than the native proteins were present in the samples from corpora lutea of late pregnancy, indicative of degradation of the native enzymes. It is concluded that corpora lutea of sheep during late pregnancy have a much lower enzyme capacity for steroidogenesis than do those of the oestrous cycle (mid-luteal phase) due to a reduction in the content of cytochrome P-450scc and adrenodoxin. The reduction in the levels of steroidogenic enzyme proteins appears to be unspecific and probably reflects an overall demise in mitochondrial functions.