The function and morphology of adrenal zona-fasciculata (ZF) mitochondria were studied in 4-, 10- and 16-month-old rats, since in this species ageing causes a marked decline in glucocorticoid secretion coupled with high levels of circulating ACTH. Dispersed intact ZF cells displayed a significant age-dependent impairment of their basal pregnenolone (PREG) secretion, but isolated ZF mitochondria showed an increased capacity to convert cholesterol to PREG (the first rate-limiting step of steroid synthesis). These data are in keeping with the contention that the age-related deficit of rat ZF secretion is located prior to the activity of intramitochondrial cholesterol side-chain cleaving enzymes (cytochrome-P450scc). Stereology showed a notable age-dependent increase in the number of mitochondria per unit cell-volume, coupled with a marked decrease in their average volume. The width of the mitochondrial intermembrane space remained unchanged, but its average volume strikingly decreased. This last finding fits well with the enhanced capacity of mitochondria to produce PREG, since intermembrane space is an aqueous barrier to the translocation of free cholesterol from the outer membrane to the cristae, where cytochrome-P450scc is located. In conclusion, the hypothesis is advanced that all these age-related functional and morphological mitochondrial changes are an ACTH-dependent compensatory response enabling ZF cells to partially counteract their decreased glucocorticoid secretory capacity, which in turn is due to the impaired utilization of intracytoplasmic stores of cholesterol esters.