Liver biopsies from two groups of women receiving oral contraceptives from 1 to 6 months and from 12 to 30 months, respectively, were examined by electron microscopy. The fine structure of the liver cells from the first group appeared normal, except for an increase in the amount of smooth endoplasmic reticulum. In the second group, however, there were many cases with striking changes-altered shape, increase in size, and presence of paracrystalline inclusions-in the mitochondria. No significant correlation between mitochondrial changes and alterations in liver function was detected.