Progesterone secretion is regulated by different mechanisms in large and small steroidogenic ovine luteal cells. Large cells secrete approximately 7-fold more progesterone in an unstimulated state than small cells. Since cholesterol side-chain cleavage, which is catalyzed by an inner mitochondrial membrane enzyme complex, is a major rate-limiting step in progesterone synthesis, mitochondrial components were quantified in the two steroidogenic cell types throughout the estrous cycle. Corpora lutea collected on Days 4 (n = 4), 8 (n = 4), 12 (n = 5), and 16 (n = 6) of the estrous cycle were prepared for electron microscopy. Volume densities of cell types within corpora lutea and mitochondrial densities within cell types were estimated by point-counting; nuclear and cytoplasmic volume densities were estimated by planimetric analysis. A total of 570 micrographs (magnification 5300 X) were analyzed. Large cell volume density was unchanged during the cycle (35 +/- 1%) while small cell volume density increased (p less than 0.05) from 13 +/- 1% on Day 4 to 20 +/- 3% on Day 12. Large cell mitochondrial volume density increased (p less than 0.05) from 13 +/- 1% on Day 4 to 23 +/- 1% on Day 16 accompanied by an increase in cytoplasmic volume density such that nuclear to cytoplasmic ratio increased (p less than 0.05) from 1:14 to 1:34 between Days 4 and 16. Small cell mitochondrial volume density increased from 11 +/- 1% on Day 4 to 14 +/- 1% (p less than 0.05) for the rest of the cycle while the nuclear to cytoplasmic ratio remained at 1:14.(ABSTRACT TRUNCATED AT 250 WORDS)