The incorporation of radioactivity derived from [2-14C] riboflavin into the flavins of rat liver mitochondrial outer membranes was studied. These membranes were found to contain about 0.6 nmol of non-covalently bound flavins per mg protein; the majority is in the form of FAD (73%) and FMN (24%). The membranes also contain about 1.5 nmol per mg of covalently bound flavins. After labeling, radioactive flavins appeared in the non-covalently bound flavins for about 4 h. Most of this radioactivity was in FAC (77%). Neither the rate nor extent of this labeling was affected by cycloheximide (1 mg/kg) administered 30 min prior to the radioactive riboflavin. With the covalently bound flavins, radioactivity was incorporated into the coenzymes for at least 18 h, but the rate of incorporation was much slower. After cycloheximide, radioactive flavins continued to appear in covalently bound flavins for about 2 h, but then stopped. Labeling of both types of flavins after [14C] riboflavin was considerably slower than the incorporation of [3H] leucine into outer membrane proteins. These results suggest that with flavoproteins from the mitochondrial outer membranes, the incorporation of flavins occurs after synthesis of the various apoenzymes is complete.